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Principles of Zoology. 



— A — 



GUIDE FOR BEGINNERS, 



RICHARD C; SCHIEDT, 

Professorrof Natural Sciences in Franklin and Marshall College, 
Lancaster, Pa. 







LANCASTER, PA.: 
EXAMINER PRINTING HOUSE. 

1892. 



COPYRIGHT BY RICHARD C. SCHIEDT, 
IvANCASTER, PA. 



.S3-6 



PREFACE. 



This little book is intended for college students, to 
whom but little time is allotted for the study of the 
natural sciences. It is to be a companion to the general 
laboratory work outlined in the elementary text-books 
on practical zoology. Its contents are based upon the 
larger text-books of Professors Arnold Lang, Berthold 
Hatschek and Korschelt and Heider, which present the 
latest results of morphological research not yet published 
in the English language. We adopted Arnold Lang's 
classification, because it is the most complete of the latest 
systems. The embryological element preponderates on 
account of its importance for modern thought in general. 
Every college graduate ought to be acquainted with the 
fundamental principles of a science which has revolu- 
tionized the philosophic systems of the past, and given 
rise to a more rational treatment of the various disci- 
plines of a college curriculum. 

Special thanks are due to Dr. John A. Ryder, Professor 
of Comparative Embryology, at the University of Penn- 
sylvania, for valuable assistance rendered in the critical 
reading of the proof. • 

R. C. S. 

Lancaster, Pa., November, 1892. 



ERRATA. 



Page 17, line 1: species instead (species). 

Page 30, line 22: omit ' ' always and." 

Page 30, line 24: essential part of the egg instead egg- 
cell. 

Page 89: heading ought to be Cnidaria. 

Page no, line 8: Archiannelida instead Archiamelida. 

Pages 1 19-143: heading on both sides ought to be 
Vermes instead Chsetognatha. 

Page 121, line 13: median line this instead medium 
live the. 

Page 226, line 3, from below: Opisthobranchiata in- 
stead Ophistobranchiata. 

A number of minor oversights are here omitted. 



CONTENTS. 



PAGE. 

Introduction 5 

Protozoa 19 

Monera 19 

Sarcodina 19 

Flagellata 21 

Gregarinida 21 

Infusoria 22 

Suctoria 22 

Catallacta 22 

Metazoa, General remarks on 29 

Structure of egg . 30 

Formation of egg 3T 

Cell division 32 

Segmentation 34 

Histology 44 

Modes of Reproduction 66 

Theories of Heredity 71 

Cceleuterata 76 

Gastrseada 76 

Porifera . 77 

Cnidaria 81 

Ctenophora 87 

Plathelminthes 95 

Turbellaria 95 

Trematoda 96 

Cestoda -97 

Vermes 104 

Nemertini 104 

Nematbelmia 105 

Annulata 108 

Prosopygii 112 

Rotatoria , . 116 

Chaetognatha 116 



11 CONTENTS. 

PAGE. 

Arthropoda 167 

Branchiata 167 

Crustacea 167 

Trilobita 177 

Gigantostraca 178 

Hemiaspidse 178 

Xiphosura 178 

Pantopoda .* . . 179 

Tracheata 180 

Protracheata 180 

Antennata 180 

Arachnoidea 189 

Mollusca 221 

Amphineura 222 

Gasteropoda 223 

Scaphopoda 230 

Lamellibranchia 230 

Cephalopoda 234 

Echinodermata 256 

Crinoidea 256 

Echinoidea 258 

Ophiuroidea 260 

Asteroidea 261 

Holothuroidea 263 

Enteropueusta 275 

Tunicata 279 

Ascidiae 279 

Thaliacea 282 

Vertebrata ' 290 

Cephalochorda 290 

Acrania 290 

Craniata 291 

Agnatha 291 

Pisces 291 

Batrachia 294 

Monocondylia . 295 

Mammalia 297 



INTRODUCTION. 



§ I. Plant and Animal. The aim of Zoology is the 
study of animal life. The question arises: What is an 
animal ? The answer to this question is simple, when 
dealing only with the more highly developed organisms; 
it is extremely difficult, when dealing with the lowest 
forms of life, because the starting point of all organic life 
and formation, is the cell. The simplest organisms, the 
lowest animals and plants, are cells. Every larger 
animal is at the beginning of its individual existence a 
cell, and every higher organism is composed of cells, 
which are the result of the reproduction of one cell.* 

The cell is the organic individual of the first order. 

Excepting the lowest organisms, a cell generally mul- 
tiplies and forms communities or states, which thus com- 
pose individuals of a higher order. Every higher organ- 
ism, every bird, every fish, etc., is such a community of 
cells. In it the closely connected cells divide the com- 
mon labor, one assuming the discharge of one function, 
another of another function, according to its particular 
ability. 

Every cell consists of two essential constituents: (i) 
the protoplasm; (2) the nucleus. The latter may be con- 
sidered as a special differentiation of protoplasm. Chemi- 
cally considered, protoplasm is a complex albuminous 
carbon compound, not yet sufficiently analyzed, whose 
constituents constantly change during life, although only 

*See Arnold gang's "Die Zelle." 



6 INTRODUCTION. 

within infinitesimally small limits. It is in a condition 
of sluggish motion and capable of swelling. The nucleus 
is a chemical and physical differentiation within the 
protoplasm. It is an essential part of the cell, in whose 
reproduction it plays an important part. Recent observa- 
tions prove that the removal of the nucleus destroys the 
cell: The addition of a nucleus to a non-nuclear plasma 
portion produces characteristic phenomena, which other- 
wise would not have arisen. 

There are minute lumps of protoplasm, representing 
the simplest organisms, in which as yet no nucleus has 
been observed. Should the non-nuclear character of 
these beings be proved, they would have to be ranked 
below the cell, namely among the cytodes. Haeckel has 
classified these simplest organisms as Monera. 

A frequent, however not essential, part of the cell, is 
the cell skin or membrane, an excreted product of proto- 
plasm, which serves as an external protection or sup- 
port. Such a membrane can likewise originate through 
the hardening and metamorphosis of the peripheral 
layers of protoplasm. 

A single cell (one-celled organism, ovum) is originally 
fitted for all those activities and functions, which are 
comprehended under the idea of life. 

The life of a cell manifests itself in the simplest, most 
undifferentiated case: 

i. In motion. Protoplasm is contractile. Its finest 
visible parts can exchange position. The cell is capable 
of changing its form and position in space. 

2. In irritability. The cell reacts through such mo- 
tions upon effects from the outside. 

3. In metabolism. Through the activity of life the 
substance within the cell is used up, decomposed. Tfye 



INTRODUCTION. 7 

useless material is excreted (excretion). Through the 
reception of food new substances are added to the cell. 
These are digestible, when they can be assimilated by 
the chemical action of the cell, and changed into con- 
stituent parts of protoplasm (assimilation). If this is 
chemically impossible, these substances are indigestible, 
and are again eliminated from the body. 

4. In growth. Through nourishment more proto- 
plasmic parts may be formed than there are originally 
present. As a consequence, the cell increases in mass 
and size; it grows. 

5. In reproductio7i. It may be supposed that the size 
of the cell is individully limited. If it grows beyond the 
individual measure, it divides into two cells. Reproduc- 
tion by division. Bach of the two parts has the same 
physical and chemical properties as the mother-cell. 
Simplest case of heredity. Through growth the daughter- 
cell reaches the size of the mother-cell. 

Now, since the cell is the starting point, both for 
animals and plants, it is easily seen, that there ca?i be no 
distinct line of demarkation between the two kingdoms in 
their lower forms. Haeckel has, therefore, established an 
intermediate kingdom, consisting of the lowest organ- 
isms, the kingdom of Protists. However, there exists 
not even between these protists on the one hand and the 
animals and plants on the other, a sharp line of distinc- 
tion. Chiefly in the method of taking in food, some of 
the protists approach more nearly the plants, others more 
nearly the animals. The latter are called Protozoa in 
contra-distinction from all the other animals, the Metdzoa. 

§ II. It is very important to note that the foregoing 
results are of comparatively modern date. To under- 
stand and appreciate them well, it is necessary to know 



8 INTRODUCTION. 

something of the historic development of Biological Science. 
Aristotle (4th century B. C.) is generally acknowledged 
as the founder of this science. He wrote treatises on the 
"Reproduction of Animals," "Parts of Animals, and 
History of Animals." The last named is only incom- 
pletely preserved. These titles alone indicate the philo- 
sophic comprehensiveness of the work, touching even 
then upon the embryology, morphology and physiology 
of animal life. He divided the animals into eight 
groups: 

Animals with blood, Vertebrates — 

1. Viviparous animals, (four-footed, including whale); 
2. Birds; 3. Oviparous, four-footed animals; 4. Fishes. 

Animals without blood, Invertebrates — 

5. Soft animals (Cephalopoda). 6. Soft animals with 
shells; 7. Insects; 8. Shelled animals, (Echini, snails 
and mussels). 

The two main divisions rest, of course, on an erro- 
neous idea since all animals have blood, the inverte- 
brates only -lacking red corpuscles. This system reigned 
supreme till the end of the Middle Ages. Even the 
modern systems since Ray and Linnaeus are based 
upon it. It was, however, not until the renaissance of 
the sciences in the 16th century that the works of 
Aristotle again came to the front, and the desire for 
experimental independent observation was aroused. 
The splendid discoveries of Harvey, Kepler and Newton 
in physics, of Swammerdam, Malpighi, L,eeuwenhoek, 
Hamm, and others in natural history, offered a prepara- 
tion for the labors of Carl Linnceus (1 707-1 778), whose 
system forms the starting point for all modern systems. 
Ray, before him, had introduced the conception of species 
and the consideration of anatomical characters as the 



INTRODUCTION. 9 

basis of classification; however he lacked methodical 
arrangement. The importance to the development of 
science of Linnaeus' work depended entirely on his acute 
sifting and exact division of the then existing facts, and 
on the introduction of a new method of more certain 
diagnosis, nomenclature and arrangement. He looked 
upon the whole animal world as a single ascending series 
of forms, and advanced the doctrine of the uni- serial 
arrangement of the animal kingdom, erecting for groups 
of different value, a number of categories based on the 
ideas of species, genus, order, class. Every animal 
received two names taken from the Iyatin language, the 
generic name, which was placed first, and the specific 
name Thus, by the introduction of binary nomen- 
clature, he created a systematic framework, not only for 
the facts known then, but for all the discoveries of the 
future. His great work, " Systema Naturae," is noth- 
ing more than an exhaustive catalogue of all the then 
known specimens of animals, plants and minerals, with 
a statement of their most remarkable characteristics. 
Thus his systems of classification, both in botany and 
zoology, are artificial, resting merely on isolated features 
of internal and external structure. His division of 
animals into six classes is founded on the structure of 
the heart, the condition of the blood, manner of repro- 
duction and respiration. 

Ctivier's conception of the different structures of ani- 
mals opened up a new epoch of zoological systems. 
George Cuvier, born at Mompelgard, in 1769, Professor 
of Comparative Anatomy at the Jardin des Plantes in 
Paris, published his investigation, especially in" his 
" Lecons d'Anatomie Comparee" (1805). In his new 
and essentially changed classification, he made the first 



I O INTR ODUCTION . 

serious attempt to build up a natural system, based 
entirely upon the idea of the correlation of parts. Recog- 
nizing the reciprocal dependence of the individual 
organs, he found, by comparing the organizations of 
many different animals, that the important organs are 
the most constant, and that the less important vary most 
in their forms and development, and are not even uni- 
versally present. Thus he became convinced that there 
were in the animal kingdom four main types, "general 
plans of structure, on which the respective animals ap- 
pear to be modelled." These four groups were the 
Vertebrata, Mollusca, Articulata and Radiata. We may 
therefore call Cuvier's doctrine, the doctrine of the parallel 
Series in opposition to the School of Natural' Philosophy 
in France, led by Geoffrey St. Hilaire, who maintained 
the unity of the plan of animal structure. Cuvier's 
principles met with the more undivided assent since they 
seemed to be affirmed by Carl Ernst von Baer's em- 
bryological work. In our times, however, Cuvier's view 
has experienced an essential modification in favor of the 
natural philosophers. Comparative anatomy found as- 
cending grades of organization in the vertebrates of the 
present; paleontology discovered a corresponding grada- 
tion in the vertebrates of the past; and embryology re- 
vealed the same serial gradation in developmental stages. 
The discovery of this most remarkable parallelism be- 
tween the three series, the anatomical, the paleontolog- 
ical and the embryological, is one of the most brilliant 
in the whole history of biology, and one which is to be 
placed to the credit of Louis Agassis. The absolute in- 
dependence and isolation of each group must be given 
up. But just as the transitional forms between animals 
and plants cannot abolish the distinction between these 



Linnaeus' System. 



Cuvier's System. 



V Siebold's System. Leuckart's System. 
1845- l848 - 



Claus' System. 



Haeckel-Hatschek. 
1888. 



1 CI. Mammalia. 
Ord. Primates, 
Ferje, Glires, Pe- 
cora, Belluie.Cete 

2. CI. Aves. 

Ord. Accipitres, 
Picae, Anseres, 
Gralte, Gallinae, 
Passeres. 

3. CI. Amphibia. 
Ord. Reptiles, Ser- 

pentes, Nantes. 

4. CI. Pisces. 

Ord. Apodes, Jugu- 
lares, Thoracici, 
Abdominales. 

5. CI. lusrrta. 

Ord. Col eoptera, 
Hemiptera, Lepi- 
doptera, Neurop- 
tera, Hvmenop- 
tera, Diptera, 
Aptera. 

6. CI. Vermes. 
Ord.Intestina.Mol- 

lusca, Testacea, 
I,ithophyta, Zoo- 
phyta. 



2. CI. Aves. 

3. CI Reptilia 

4. CI. Pisces. 

2. Cycle: Mollusca. 

1. CI. Cephalopoda. 

2. CI. Pteropoda. 

3. CI. Gasteropoda. 

4. CI. Acephala. 

g CI. Brachiopoda. 
6. CI. Cirrhopoda. 

3. Cycle: Artirulala. 

1. CI. Annelides. ' 

2. CI. Crustacea. 

1. Sect. Malacos- 
traca. 

2. Sect. Eutomos- 
traca. 

3. CI. Arachnides. 

4. CI. Insecta. 
Ord. Myriapoda, 

Thysamira.etc. 
. . bipterea. 

4. Cycle: Radial a. 

1. CI. Echiuoderma- 
ta. 

2. CI. Intestiua. 
(Ord. Nematoidea 

Farenchymata) 

3. CI. Aealephae. 
(Ord. Simplices, 

Hydrostatics:). 

4. CI. Polypi. 

5. CI. Infusoria. 
(Ord. Porifera, 

Homogenea.) 



I. Protozoa. 

1. CI. Infusoria. 

2. CI. Rhizopoda. 

II. Znophy 



3. CI. 



Polypi (An- 
3a, Bryozoa.) 

4. CI. Acalepha:. 

5. CI. Echinoderm- 
ata. 

III. Vermes. 

6. CI. Helminthes. 

7. Cl.Turbellarn. 

8. CI. Rotatorii. 

9. CI. Annulati. 

IV. Mollusca. 

10. CI. Acephala. 
(Tunicata, Brach- 

iopoda and 
Lamellibrau- 
chiata.) 
11 CI. Cephalo- 
phora. 

12. CI. Cephalopoda 

V. Arlluopoda. 

13. CI. Crustacea. 

14. CI. Arachnida. 

15. CI. Insecta. 

VI. Verlebrata. 



\. Protozoa (to be 1. 

separated from the 2. 

other animals.) 
[ Coelenierata. 

1. CI. Polvpi. 3- 

2. CI. Acalephae. 
[I. Echinodermata. 

3. CI. Pelmatozoa. 4. 
(Ord. Cyst idea, 

Crinoidea.) 

4. CI. Actinozoa. 
(Ord. Echinida, 5. 

Asterida.) 

5. CI. Scytodermata! 
(Ord. Holothuritei 

and Sipunculi-!6. 
da.) 7' 

[II. Vermes. 

6. CI. Anenterati. 
(Ord. Cestodes, 

Acantocephali.JS. 

7. CI. Apodes. 19. 
(Ord. Nemertini, 

Turbellarii, Tre-; 
matodes, Hirud- 
inei.l 

8. Cl.Ciliati. 
(Ord. Bryozoa, 

Rodiferi.) 

9. CI. Annelides. 
(Ord. Nematodes, 

Lumbricini, 
Branchiati.) 

IV. Arthropoda. 

10. CI. Crustacea. 

11. CI. Insecta. 

V. Mollusca. 

12. CI. Tunicata (!) 

13. CI. Acephala. 

14. CI. Gasteropoda 

15. CI. Cephalopoda 

VI. Vertebrata. 



Protozoa. 
Coelenierata. 

(Spongiaria, Cm- 

Echinodermata. 
Appendix: Entero- 
pueusta. 

Vermes (Platyhel- 
minthes.Nemathel- 
minthes, Annelides 
Rotatoria.) 



A. Protozoa. 

B. Mctazoa. 

a. Protaxonia(=Ccelen- 
terata.) 

I. Type, Spongiaria. 

1. Clad. Spongiaria. 

II. Type, Cnidaria. 

2. Clad. Cnidaria. 

1. CI. Hydrozoa. 

2. CI. Scyphozoa. 
App. Plauuloidea. 



'Arthropoda (Crust-: HI. Type Clr,,„;,/,,u ,, 
acea, Arachnoidea.l 3 .Clad.Cteiiophor< 

Ouychophora, My 



riopoda.Hexapoda) 

Mollusca. 

Molluscoidea (Bry- 
ozoa [endoprocta; 
and ectoprocta]j 
Brachiopoda.) 

Tunicata. 

Vertebrata. 



App ; 



Heter 

(=Bilateria.) 
. Type, Zvgoneura. 

I. Subtype: Autos- 
colecida. 

4. Clad. Scolecida. 

1. CI. Platodes. 

2. CI. Rotifera. 

3. CI. Endoprocta 

4. CI. Nematodes 

5. CI. Acantho- 
cephali. 

Nemertini. 
2. Subtype: Aposcoleci- 
da. (Metanephrido- 
zoa.) 

5. Clad. Articulata. 

1. CI. Annelida. 
App. Sipunculoidea. 
App. Chaetognathi. 

2.C1.0nychophora 
3. CI. Arthropoda. 

6. Clad. Tentaculata 
(=Molluscoidea.) 
1 CI. Phoronida. 

2. CI. Bryozoa 
(ectoprocta.) 

3. CI . Brachiopoda 

7. Clad. Mollusca. 

1. Subclad. Am- 
phineura. 

2. Subclad. Con- 
chifera. 

V. Type. Ambulacra- 
lia. 

8. Clad. Echinoder- 
mata. 

9. Clad. Enterop- 
ueusta. 

VI. Type, Ckordonii. 
10. Clad. Tunicata. 

II. Clad.Leptocardii 
12 Clad. Vertebrata. 

1 Subclad. Cyclo- 

stomata. 
2. Subclad. Gna- 

thostomata. 



ek. 



a. 
iria. 



oa. 
zoa. 



ora. 
Lora 



ttos- 

ida. 
s. 
i. 

pcta 
des 



leci- 
ido- 



lora 

)da. 

lata 

lea.) 

fla. 

i o a 

loda 

ca. 

ft.m- 

:on- 

a a- 

der- 

rop- 

Y. 

ta. 

rdii 

ata. 

clo- 



INTRODUCTION. 1 1 

two most important conceptions of the organic kingdom, 
so the existence of such transitional forms does not in 
any way affect the value of the idea of groups and types 
as the chief divisions of the animal system; but only 
renders it probable that the different groups have 
developed from a similar or common standing- point. It 
was left for Charles Darwin to show that the coincidence 
pointed out by Agassiz between the geological succes- 
sion, the embryonic development and the zoological 
gradation held also in the geographical distribution of 
animals in the past and the present, and to found the 
interpretation of the fact now universally accepted. 
Descent was seen to be " the hidden bond of connection," 
and embryonic development came to be regarded as the 
epitomized history of ancestral development. "In two 
or more groups of animals, however* much they may 
differ from each other in structure and habits in their 
adult condition, if they pass through closely similar 
embryonic steps, we may feel assured that they are all 
descended from one parent form." So we find that the 
higher groups are genetically to be derived from the 
worms, etc. Thus embryology came to have a higher 
value in classification than anatomy, and to take the 
place assigned to it by C. von Baer more than a half 
century ago as ■ ' the true torch-bearer in the investiga- 
tion of organic bodies." 

Under such circumstance we find it convenient to 
place the different methods of classification side by side, 
in order to show the difference of the various principles 
which govern the methods up to the present time. (See 
the table.) It will be noticed that the last ones represent 
no longer parallel series, but a genealogical tree. In the 
book itself the latest classification of Invertebrates as 



12 INTRODUCTION. 

given in Prof. Arnold Liang's "Iyehrbuch der Vergleichen- 
den Anatomie " has been adopted, whilst the classifica- 
tion of Vertebrates is taken from Prof. K. D. Cope's 
" Synopsis of Vertebrate Palaeontology." 

The above table shows that Hatschek's classification is 
decidedly distinct from that of his predecessors. It is 
chiefly based upon modern embryological investigation, 
first begun (1870) by Kowalevsky. Haeckel was the 
first one to point out their great importance for classi- 
fication. The systematic significance of his gastraea 
theory culminates in the following important fact: "The 
w T hole animal kingdom is divided into two great series 
separated by the gastrula; on the one side, the Protozoa 
(Urthier); on the other, in sharp contrast, the six higher 
animal branches, which may be termed the Metazoa or 
Blastozoa" The morphological significance of this ap- 
parently simple division may be comprehended in this 
statement. All higher animal branches are derived from 
the gastraea, a form common to all, which possesses a 
monaxial body, whose cavity opens at one pole through 
the primitive mouth, and whose body wall consists of 
two cell strata the exoderm and the endoderm. Al- 
though Haeckel' s attempts to define the relations of the 
different types more closely, partly failed, a whole series 
of similar investigation followed, some of which will be 
of lasting value, e. g., the hypothesis, that the verte- 
brates are derived from the annelids, established by 
Semper. 

Principles of Morphology.* 

Biology, or the science of the organisms, comprises 
two sub-groups: Physiology, dealing with the functions 

*See Hatschek's "Principien der Morphologic" 



INTRODUCTION. 1 3 

ft 

of life, and Morphology, dealing with the structure (in- 
ternal and external) of organisms. 

Physiology either treats of the special relation of the 
form of an organism to its physiological functions, when 
it is called physiological anatomy, or it sets forth general 
principles which govern the formation of typical organi- 
zation, e. g., the functions of the bilateral or the radial 
structure of living beings, when it is spoken of as physi- 
ological Morphology or it may have reference to the laws 
of mechanics, nourishment or growth in the develop- 
ment of structures; as such it is termed physiological 
embryology . 

Morphology, in the special sense of the word, or Ge?te- 
alogical Morphology is the name of that science which 
exclusively treats of the structure of organisms, and their 
relation to common ancestral forms. 

Morphology may therefore be subdivided into compar- 
ative anatomy and comparative embryology (comp. onto- 
geny). 

PRINCIPLES OF COMPARATIVE ANATOMY. 

The theory of descent firmly established the principles 
of Comparative anatorc^. The distinction between ho- 
mology and analogy is here of special importance. 

The term homology is used to designate identity of 
descent of certain organisms, i. e., in the sense of 
homophyly. Analogy is used to designate the similarity 
of structure in organisms of different descent, but of the 
same physiological function, e. g., homologues are the 
anterior extremities of all vertebrates, although they may 
physiologically vary as fins, feet or wings. Homologous 
are the swimming bladder of the fish and the lung of 
higher vertebrates. Analogous, however, are the feet of 



14 INTRODUCTION. 

a vertebra, and those of an insect, or the wings of an 
insect and those of a bird; so the wings of bats are only 
homologous as anterior extremities ;" in their structure 
as wings they are analogous. Further, the lungs of a 
vertebrate and the tracheae of an insect are analogous ; 
so also the highly developed eye of a cephalopodon and 
that of a vertebrate. 

Homodynamous organs are organs of the same kind 
repeated in the same animal, e. g., the anterior and pos- 
terior or extremities of a vertebrate animal; homotypi- 
cal are the organs repeated on either side of a bilateral 
or in the rays of a radial animal. 

principles of comparative embryology (biogenetic 

law). 

Distinctions between homology and analogy are here 
of even greater importance than in comparative anatomy. 
It must be maintained of all plants and animals save the 
simplest unicellular organisms, that each one is repre- 
sented during life not merely by one fixed structural con- 
dition, but by a series of such conditions, although one 
is always predominant for a longer period, generally that 
in which reproduction takes place. This is especialty 
true of the multicellular organisms, where the unicellu- 
lar ovum gradually differentiates into a complex variety 
of tissues and organs. Comparative embryology, there- 
fore, treats not of comparisons of single structual condi- 
tions, but of series of such conditions. Already at the 
beginning of the 19th century we find it stated that the 
higher animals pass in their development, through stages 
which in their structure correspond to certain lower ani- 
mal forms. C. B. v. Baer elaborated that idea further, 
and the theory of descent created new interest in it, until 



INTRODUCTION. 1 5 

Fritz Mutter, in 1864, found what we may call the for- 
mula for comparative embryology which is valid to this 
very day. 

"The historical manuscript (of the development of the 
ancestors) preserved in the history of development (of 
individuals) gradually fades with the steadily straighten- 
ing course of development from the egg to the complete 
animal; and it is frequently forged through the struggle 
for existence, which the larvae of independent modes of 
living have to undergo. The primitive history of the 
species (phylogenesis) will be so much more complete in 
the history of its development (ontogenesis), the larger 
the series of immature condition is through which it has 
to pass pari passu, and so much more faithful the less the 
mode of living among the different stages differs, and 
the less it is found, that the peculiarities of the different 
immature conditions are retrogressions from later epochs 
or are independently required. (Fur Darwin, Leipzig, 
1864, p. 77. 78.)" Thus the embryonic character de- 
pends upon its ability of further development. Ernst 
Haeckel deserves the credit of having been the first one 
who introduced this theory methodically, e. g., in his so- 
called "Gastraea Theory," where he endeavors to ex- 
plain phylogenetically the first stages of development, 
and to prove the relation of all metazoan organisms to a 
common ancestral form, the gastraea. 

The theory of the parallelism of individual develop- 
ment with the historical development of the species was 
termed by Haeckel ' 'Fundamental Law of Biogenesis," 
and expressed in the sentence: Ontogeny is a short re- 
petition of Phytogeny. He divides the phenomena into 
palingenetic which repeat the phenomena of a once de- 
veloped ancestral form, and into ce?wge?tetic phenomena 



1 6 INTRODUCTION. 

which arose by adaption to the embryonic or larval life. 
Hatschek, a pupil of Haeckel, has further developed 
and modified these theories, reaching the conclusion that 
the phylogenetic changes occur in most cases through an 
addition of new stages to the end of the ontogenetic 
series of forms. He explained this by maintaining that 
since, in his opinion* only those new characteristics are 
transmitted, which originate through the variations of 
the reproductive cells, ''overreaching varieties" must be 
accepted, which consist in a continuation of the ontogen- 
etic series of forms. Therefore cenogenetic characters of 
ontogeny are just as important for the investigation of 
animal relationship as the palingentic characters. Care 
must be taken, however, that we do not go too far in 
these theories, making every larval or embryonic form, 
which is characteristic for a whole group of animals, the 
representative of a similar phylogenetic stage. 

Meaning of the System. Categories of the System. 

It is easy to see that the system is based upon a simi- 
larity of organisms which is different in degree. Know- 
ing that descent is the cause of similarity, we can easily 
understand that the actual basis of the system must be 
the genealogy of the organisms. The system is an ex- 
pression of our views, referring to the relationship of 
organisms, a relationship based upon actual descent. 
The arrangement of the system, according to larger di- 
visions and smaller subdivisions, corresponds to the 
branches and twigs, of the genealogical tree. The 
usual divisions or catagories are : kingdom (regnnni), 
subkingdom (subregnum), also 'called cycle, type or 
phylum; class (classis); order (ordo); family (familia); 

*Weissmanti. 



INTRODUCTION. 1 7 

genus, (species). The binary nomenclature of Linnaeus 
requires only the two last divisions in naming an organ- 
ism. 

MEANING OF SPECIES. 

The similarity of organisms increases the deeper we 
descend in the categories. The idea of species, how- 
ever, is not only based upon similarity of forni, but 
much more upon the relation of sexual intercommunion 
which takes place within the species. 

i . Similarity of the individuals of one species. Identity 
of cycles of forms, not of single forms, is the ontogenetic 
requirement. Complications arise through alternations of 
generation when the individual is repeated not in the 
cycle of one, but of two or more generations ; further, 
through polymorphism, when one male is complemented 
by two or three females; or one female by two male forms; 
also through functional polymorphism, which is, e. g., 
found among the bees, where males, females and work- 
ers exist, or among the termites and ants, where there are 
more families of workers. 

Temporary changes conditioned by environment are, 
according to Nageli : Variations, and as such non- 
essential. Transmitted differences in the same individ- 
ual are varieties. If they occur only in one individual, 
they are individual varieties; if in a large number of indi- 
vidual, race varieties, or simply race or subspecies. The 
races are considered as the beginnings of the formation 
of new species. It is however, a matter of course that 
under certain conditions variations and varieties may 
belong to one and the same species. 

2. Perfect fertility within the species. Cuvier first em- 
phasized the necessity of perfect fertility as an important 



1 8 INTRODUCTION.. 

criterion of the species in the crossing of their individu- 
als. Perfect fertility is continuous through many gen- 
erations among the individuals of the same species. 
The individuals of different species of the same genus 
are only imperfectly reproductive, i. e., their fertility 
decreases and disappears within the next generation or 
generations. An exception is the crossing of hare and 
rabbit. 

The species is therefore not simply a sum of individuals \ 
but 7'ather a physiological unit formed by these individuals. 



PROTOZOA. 



I Class: Monera. Simplest organisms. Small lumps 
of protoplasm of various, changeable form, in which 
nuclei have not yet been discovered. L,oconiotion and 
nourishment by means of either obtuse (amoeboid) or 
pointed threadlike processes (pseudopodia). Repro- 
duction by fission or budding. All monera live in 
water. Protamceba, Myxodiction, Protomyxa. 

II Class: Sarcodina. Unicellular organisms with 
one or more nuclei. Locomotion and nourishment by 
means of changing, shorter or longer, threadlike, non- 
ciliating processes (pseudopodia). Reproduction by fis- 
sion or budding. 

i. Subclass: Amcebina. Sarcodina naked or enclosed 
in shells of changing form. Locomotion and nourish- 
ment effected through the flow of the body and the 
formation of changing processes of short, leafllike form. 
Contractile vacuoles mostly present. Amoeba proteus, 
Arcella vulgaris, Difflugia pyriformis, Quadrula symme- 
trica, Hyalosphenia lata. 

2. Subclass: Rhizopoda. Sarcodina whose protoplasm 
secretes a shell of varying form; at first mono-axillar, 
chitinous, mostly calcareous. Nourishment and locomo- 
tion through pseudopodia, which often fuse, forming a 
network. Contractile vacuoles mostly absent. 

A. Imperforata. Shells one or many- chambered, hav- 
ing one or two larger openings through which proto- 



20 PROTOZOA. 

plasm and pseudopodia protrude, not pierced by fine 
pores. Miliola, Lituola, Gromia oviformis. 

B. Perforata. Shells one or two-chambered, perforated 
in order to protrude pseudopodia. Globigerina, Rotalia. 

3. Subclass: Heliozoa. Sarcodina, naked or enclosed 
within silicious shell, with fine, somewhat stiff pseudo- 
podia, protruding in all directions. Contractile vacuoles 
mostly present in varying numbers. Actinophrys sol.,' 
Adinosphcerium. Acanthocystis, Clathrulina. 

4. Subclass: Radiolaria. Body separated by a capsu- 
lar membrane; originally round or oval, into an external 
(extracapsulum) and an internal nuclear part (central 
capsule). The extracapsulum consists of non-nuclear pro- 
toplasm and a slimy envelope (calymma), whence their 
pseudopodia protrude in all directions. Skeletons of 
extraordinarily variable form, consisting of silica or a 
chitinous organic substance, are rarely absent. Wonder- 
fully rich in differentiations of form. Without contrac- 
tile vacuoles. Unicellular algae found with them. 
Family of Polycettaria forms colonies. 

A. Porulosa. Central capsule spherical, without main 
opening, with innumerable fine pores. 

I. Spumellaria. Nucleus central, dividing late. Skele- 
ton silicious or absent, never penetrating the intracapsu- 
lar protoplasm. Thalassicolla, Collozoum, Sphcerozoum, 
Thalassoplancta, Collosphaera Dictyastrum. 

II. Acantharia. Nucleus extra centric, dividing early. 
Skeleton chitinous, radiating from the middle of the 
central capsul. Acanthometra, Phractaspis. 

B. Osculosa. Central capsule oval, main opening at the 
basal pole of chief axis. Skeleton silicious, always extra 
capsular. Nucleus dividing late. 

III. Nassellaria. Capsular membrane simple, a porous 



PROTOZOA. 2 1 

ring at the oral pole of the chief axis. Nassela, Cortina, 
Gornutella. 

IV. Phczodaria. Capsular membrane double, at the 
oral pole of the chief axis the osculum, which is closed 
by a radially striped cover, with a central drawn out 
opening. Pigment bodies in the calymma. Aulosph&ra, 
Azdactinium, Cannopilus, Challengeria. 

Ill Class : Flagellata (Mastigopora). Organisms of 
one cell or of colonies of one cell, rank very low, allied 
to certain fungi. Possess one or more long flagella or 
cilia. With contractile vacuoles. Reproduction by fission 
or budding, often after preceding copulation of the re- 
productive individuals. 

I Order : Flagellata s. str. During active life provided 
with flagellar exclusively. Monas, Euglena, Ckilomonas, 
Eudorina, Pandorina, Stephanosphczra, Volvox. 

II Order : Choanoflagellata. Flagellum surrounded at 
basis by funnel-like collar. Phalansterium, Salpingceca, 
Protospongia. 

III Order : Cystoflagellata. Protoplasm shows the net- 
ted structure of that of the plant cell. Noctiluca, Lep- 
todiscus. 

IV Order: Dinoflagellata (Cilioflagellata). Shelled 
forms. Outside of the freely protruding flagellum a 
second separate flagellum, moving in a special cross 
furrow of the body, formerly looked upon as a row of 
cilia. Peridinium, Ceratium. 

IV Class : Gregarinida. Parasitic protozoa of slender 
form. Always only one nucleus. Without pseudopodia, 
without cilia, without contractile vacuoles, without 
special differentiation of protoplasm, with external cell 
membrane. Reproduction by spores after copulation or 
conjugation. 



22 PROTOZOA. 

I Order: Monocystidea. Body simple. Monocystis (in 
earthworm). Urospora. 

II Order : Polycystidea. Body separated by a partition 
wall into protomerit and deutomerit. Nucleus in deuto- 
merit. Actinocephalus, Stylorhynchus, Clepsidrina. 

V Class: Infusoria (Ciliata). Unicellular, rarely 
colonized protozoa, with cilia or cilia-like continuations 
for locomotion and nutrition. Mostly with contractile 
vacuoles, mouth and anus. With nucleus of various 
forms and peculiar paranucleus (falsely nucleolus). Re- 
production by fission, frequently by conjugation. 

I Order : Holotricha. Cilia covering the whole body 
in rows. Paramecium, Trachelius. 

II Order: Heterotricha. Cilia in the region of the 
mouth grouped so as to form an adoral zone of large 
bristle or hook-like cilia. Spirostomum, Stentor, Freia, 
Balantidium. 

III Order. Hypotricha. Dorsal and ventral plane 
sharply divided. Chilodon, Euplotes, Sty /onychia, Oxy- 
tricha, Urostyla. 

IV Order: Peritricha. Body spherical or cylindrical, 
only partially ciliated. Cilia either in an adoral spiral 
or in a girdle. Vorticella, Carchesium, Epistylis, Tricho- 
dina, Strombidium, Tintinnus, Ophrydium 

VI Class: Suctoria (Acineta). Body usually without 
cilia (only when young); with knobbed tentacle-like 
processes, which serve as sucking tubes, through which 
they suck the protoplasm of other infusoria. Reproduc- 
tion by budding. Acineta, Podophrya, Dendrocometes. 

VII Class: Catallacta. Only one genus and species; 
Magosphcera planula. Free swimming in the sea. Coast 
of Norway. A spherical colony of pear-shaped cells, 
whose stems are united in the center, the external sur- 



PROTOZOA. 23 

face covered with cilia. Reproduction: the colony sep- 
arates into single cells. They sink to the bottom, be- 
come like amoeba, form a capsule and within this a new 
colony through successive fission. Afterwards the colony 
separates from the capsule. 

The protozoa are unicellular organisms or simple 
colonies of similar unicellular organisms. Frequently 
more than one nucleus appears. The morphological dif- 
ferentiation is developed to the highest degree in some 
protozoa, so that a complication of structure arises, which 
furnishes special provisions for every function of life, 
while the lowest protozoa accomplishes the same work 
with the simplest apparatus. Protoplasm flows around 
its food, assimilates, excretes, grows and divides in order 
to reproduce itself. Reproduction by fission. 

I. Protoplasm. The protoplasm of many protozoa is 
homogeneous. In the majority, however, a differentia- 
tion into a more firm, hyaline ectosarc and a more fluid, 
granular endosarc is observed. Only in the Radiolaria 
both parts are separated by a membrane with communi- 
cating openings of various contrivances. From the 
ectosarc are developed: the pseudopodia, cilia, flagella, 
sucking tentacles, cuticula, skeletons, contractile vacu- 
oles and the lasso-cells. In the endosarc are found: the 
nucleus, food vacuoles, excretion products, drops of fat 
or oil, gas bubbles and pigment granules. The endosarc 
sometimes exhibits slow circulation. The arrangement 
of the protoplasm resembles, especially in the Cysto- 
flagellata, that of the plants. Many flagellata contain 
chlorophyll or other similar pigments. 

II. Locomotion. The Rhizopoda, Heliozoa, and Radio- 
laria, move and take food through pseudopodia. There are 
two kinds of such pseudopodia: myxopodia and axopodia. 



24 PROTOZOA. 

The former are found among Rhizopoda and Radiolaria 
and are very flexible, whilst the latter are more or less 
stiff and not adapted to the formation of nets. 

Related to the pseudopodia are the flagella, vibrating 
processes of the extosarc, occurring among the Flagellata. 

Undulating membranes have been observed among the 
Flagellata. 

Cilia fine, vibrating, hair-like processes are character- 
istic for the Infusoria and the young of the Suctoria. 

At the base of the mouth of Noctiluca a large, slowly 
vibrating, ribbon- like flag ellum has been observed. 

The Gregarinida have no organs of locomotion, the 
extosarc being differentiated into contractile and not 
contractile stripes. 

The Vorticella possesses a spirally moving muscle stem. 

The myophrysks of the Acanthometridse have the power 
of contraction. 

The sticking tubes of the Suctoria are most closely 
related to the Suctoria. 

III. Membranes, shells, skeleton formations. Many pro- 
tozoa are naked. Others are covered by a chitinous 
membrane (e. g. Gromia). The infusoria are enveloped 
in a cuticula, which may harden into shells. The com- 
plicated shell of the Dinoflagellata consists of cellulose, 
felly-like envelopes also appear. The marine Rhizopoda 
carry lime shells (originally chitinous membranes). The 
Heliozoa secrete a silicious skeleton. The skeletons of the 
Radiolaria show wonderful complications; in some they 
are silicious and external, in others they are organic and 
central. The protomerit, deutomerit and epimerit of the 
Gregarinida, produced by partition walls, have already 
been mentioned. The cysts or capsules are formed for 
protection against atmospheric conditions. 



PROTOZOA. 25 

IV. Apparatus for nutrition. The organs of locomo- 
tion serve also for the apprehension of food. Only the 
Infusoria and certain Flagellata possess a mouth opening, 
mostly surrounded by cilia and only noticed when food 
is conveyed to it. The same protozoa possess also an 
anus. 

V. Apparatus for excretion. It is doubtful whether 
the contractile vacuoles really serve as organs of excre- 
tion. Their number is varying. They are wanting al- 
together in the Rhizopoda, Radiolari, Gregarinida and 
Cystoflagellata. When there is only one it generally 
has a definite position. Before and during contraction 
they move to the surface, empty their contents and 
vanish. They arise again as small droplets. Perhaps 
they also serve purposes of respiration, changing the 
water. 

VI. Trichocystes. Small bladders with lasso threads 
occur in the ectoplasm of a few infusoria and of one 
Flagellate. 

VII. Stigmata or red eye spots are found singly in 
many colored Flagellata; their function is doubtful. 

VIII. Nuclei are wanting in the monera. In the 
others there is either one or more than one present. 
They are situated in the endosarc, being either vesi- 
cular (differentiated) or homogeneous. Their form is 
very varying, especially among the Infusoria, where we 
find a nucleus and ^paranucleus. The nucleus degenerates 
during or after copulation, the new nucleus forming out 
of the paranucleus. The division of the nucleus is 
either direct or indirect. 

IX. Reproduction. A series of transitions from the 
simplest asexual to the sexual reproduction has been 
established. Reproduction by simple fission is known to 



26 PROTOZOA. 

exist among almost all the Protozoa, except among the 
Gregarinida. Budding in the simplest form differs from 
the preceding inasmuch as one part is smaller (bud) 
than the other (mother). That includes the possibility 
of numerous buds. Fission and budding occur together 
among Rhizopoda, Heliozoa, a few Gregarinida, in Noc- 
tiluca and the Suctoria. The processes of fission and 
budding are probably preceded by the conjugation (tem- 
porary combination) or by the copulation (permanent 
union) of two individuals. In a great many cases the 
buds or parts remain together and form colonies, an im- 
portant factor which reminds us of the cell-colonies of 
Metazoa. Noctiluca presents a good example for bud- 
ding. After the buds are produced each one separates 
as a swarmspore with flagellum and peculiar process. 
Very frequent is the reproduction by continued fission 
and spore formation. In the latter case the whole body 
is divided into a large number of nuclear parts called 
spores, which usually move about freely; sometimes they 
are amoeboid, or provided with flagella as swarmspores. 
These two last named forms of reproduction occur among 
encysted individuals, frequently after conjugation or 
copulation, so among the Gregarinida and the Flagellata. 
The spores frequently reproduce by division, and these 
parts become adult animals. 

Reproduction of colonies. Colonies are the result of in- 
complete division; they can reproduce themselves by 
division. Among the Radiolaria and the Flagellata 
swarmspores are formed. Among the colony forming 
Radiolaria the contents of the central capsule produce al- 
ternating spores : i. Isospores, corresponding to the usual 
spores of the other Radiolariae. 2. Anisospores, which are 
again subdivided into Microspores and Macrospores. In 



PROTOZOA. 27 

the former, development is direct, in the latter after copu- 
lation. The microspores and' macrospores are formed 
either in one and the same individual of a colony or in 
more than one. A probable alternation of generation. 
The processes of reproduction in the colony forming 
Flagellata are very interesting. In the simplest case 
every individual of a colony divides into a number of 
parts, which separate together as a daughter colony from 
the mother colony. In other cases the different parts of 
a daughter colony (gametae) separate. Every two and 
two of different size copulate. The product (zygote) be- 
comes after a while again a colony by division. In 
Eudarina and Votvox distinct sexual generations arise; 
in the former sexual generations follow asexual ones; 
they are either female (ovoid gametes) or male (spermoid 
game to) ; in the latter a division of labor occurs, inasmuch 
as only certain individuals are reproductive, and both 
sexes are produced in one and the same individual. This 
is ^genuine alternation of generations. The sexual repro- 
duction corresponds to that of the Metazoa. The female 
gamete corresponds to the ovum, the male to the sperma- 
tozoon. The formation of a new Volvox colony by suc- 
cessive fission of the zygote corresponds to the cell di- 
vision of the metazoa. Something similar has been ob- 
served among the colony forming Infusoria. 



METAZOA. 



From the protists or Protozoa are distinguished the 
higher animals or Metazoa. Their body consists of a 
large number of cells which form a community, morpho- 
logically complicated and physiologically perfected by a 
division of labor. Some of them are little above the 
Protozoan colonies. Such are, e. g., the Hydra. Their 
bodies consist of only a few different kinds of cells: of 
digestive, of neuro-muscular, of lasso and of reproduc- 
tive cells' (ova and spermatozoa). All these cells are 
indispensable to the existence of the body; if any one of 
them should be wanting the body would die. Thus the 
whole is physiologically an individual; but in contradis- 
tinction from the cell an individual of a second higher 
order, frequently called a person. Most animals remain 
in this stage of individuality. A medusa, a worm, a 
crab, a cat, are individuals of the second order. In 
some of them reproduction takes place by fission or bud- 
ding without separation of the new individuals. Thus 
an individual of the third order or a cormus is formed. 
The individuals composing such a cormus may either 
remain alike, when they bear a similar relation to the 
cormus as the cell individuals of a Protozoan colony to 
this colony; or, another division of labor may take place 
and consequently also a different formation and function 
of the structure of the individual persons belonging to 
the cormus {polymorphism). Most corals belong to the 
former, the Siphonophora to the latter class of cormi. 



30 METAZOA. 

However complicated such a community of cells may 
be, it always (asexual reproduction excepted) originates 
in one single cell, namely, in the fertilized ovum, which 
itself is the result of the union of a female with a male 
reproductive cell. This process is called sexual repro- 
duction. It presupposes on the part of one of the cells, 
at least, free motion and the power of resistance against ex- 
ternal influences. These cells are besides very small, 
and therefore present in very large numbers and capable 
of easily entering the second cell. They are the male 
reproductive cells or spermatozoa. The other cell is to 
form the new organism, and must therefore possess a 
considerable amount of food material. They are the 
largest cells, generally called female reproductive organs 
or ova.. Both are formed either in the same individual 
{hermaphroditism) or in two different individuals, male 
and female (gonochorism) . The former is the exception, 
the latter the rule. It is probable that the sexes of all 
Metazoa were originally separate. There are special 
organs of copulation to assist foreign fertilization. 

The structure of the ovum shows two distinct constitu- 
ents. The essential part, which is always and often 
alone present, is the egg cell; of secondary importance are 
the external albuminous' masses and egg covers. The egg 
cell (also called "yolk") is morphologically a cell con 
sisting of nucleus, cell body and cell membrane. The 
nucleus, also called germinal vesicle, consists of a mem- 
brane, a fibrous network and one (or more) ?iucleolus (macu- 
lae germinativse). The cell body consists of protoplasm in 
w T hich yolk substances are imbedded. They constitute the 
reserve food material, being either of a liquid albuminous 
or fatty nature, or, as in most cases, solid albuminous yolk 
granules. These are of varying shape and present in 



KGG FORMATION. 3 1 

various quantities. The cell membrane is formed by the 
egg cell itself, and found in almost all ova, except where 
a chorion or follicular membrane is developed. It is 
called yolk membrane, and possesses a various consistency. 
Both, yolk membrane and chorion, are the primary mem- 
branes, whilst many ova show secondary membranes and 
food stuff, arisen from the accessory glands of the genital 
organs. In many Metazoa several eggs are enclosed by 
a secondary membrane. 

The formation of the egg takes place in the ovaria. 
The young ovum has the structure of a simple undiffer- 
entiated cell; when ripening it separates, increases in 
size and forms primary membranes The germinal layer 
of the ovaria often resembles an incoherent plasma mass 
with numerous nuclei. Later, however, cell membranes 
form and the mass separates into distinct cells, which 
assume a spherical shape. Growth may be complete at 
this moment or continue. In young egg cells the nucleus 
is large and the protoplasm sparing. In the process of 
growth the latter increases more rapidly than the former; 
in some cases yolk substance separates from the cell; in 
others it enters the cell from neighboring follicular cells. 
Membranes are generally developed at the end of the 
growing period; the yolk membrane arises from the egg 
cell itself, the chorion from the follicle cells surrounding 
the egg cell. There are two modes of egg formation. 
Among the lower animals every epithelial cell of the 
ovarium becomes a solitary iridependent egg cell, often 
fastened by a stem to the germ layer; among the higher 
animals groups of cells arise, but only one in each group 
develops into a genuine egg cell, whilst the others fur- 
nish its food and its membrane (chorion). This is called 
follicular egg formation. 



32 MKTAZOA. 

Spermatoza are developed in the testes, which are equiv- 
alent to the ovaria; here, likewise, epithelial cells change 
into reproductive cells. Their general form resembles 
small ciliated cells, representing a head, consisting of 
nuclear substance and a small amount of protoplasm, and 
a movable tail. Animals, void of cilia, contain simple 
cone- amoeboid- or star-shaped spermatozoa without a 
tail. The epithelial germ cells of the testes first change 
into so-called mother cells (spermatoblast, spermatogony), 
which divide, forming spermatocytes; these in turn change 
into separate spermatozoa. The spermatoblast is the 
equivalent of the ovum. They are much smaller and 
more numerous than the ova, but they contribute equally 
much to the fertilized egg. The female furnishes the 
substance, the male the energy of combination. Before 
passing to this stage of the process we have to consider 
cell division in general. 

Cell division is always introduced by a division of the 
nucleus. "Omnis nucleus e nucleo." Direct cell di- 
vision takes place only among monocellular organisms; 
the cell divides into two parts by a median constriction. 
Indirect cell division (caryokinetic or mitotic) implies a 
series of characteristic nuclear changes. 

i . At the beginning of cell division two opposite cen- 
tres of attraction (asters or archoplasmic spheres) arise on 
opposite sides of the nucleus; protoplasmic particles 
group themselves around them in a radiating manner 
(archoplasmic filaments) . The chromatic (stainable) parts 
of the nucleus arrange themselves into a filamentous 
bundle along the periphery. 

2. The nuclear membrane disappears, and the chro- 
matic filamentous bundle divides into separate parts. 

3. These parts, each shaped like a bow, arrange them- 



CEl.lv DIVISION. 33 

selves in an equatorial plane between the two archo- 
plasmic spheres, their angular ends directed towards the 
nuclear centre and connected by achromatic fibriles with 
the archoplasmic centres. 

4 At the same time the archoplasmic filaments press 
forward and flatten the poles of the nucleus, until they 
have reduced the chromatic contents of the nucleus into 
aflat "plate," thus forming two cone-shaped spindles ; 
facing each other with their apices. 

5. By gradually lengthening they continue to grow 
in the same manner, and push through between each 
other, thus dividing and carrying with them the chro- 
matic bows or chromosomes, which split lengthwise and 
move one- half towards the one end of the nucleus the 
other towards the opposite end, thus forming two parallel 
plates (metakinesis). 

6. The chromosomes again change into a filamentous 
bundle, around each of which a nuclear membrane is to 
be recognized. During this process there arises on the 
surface of the mother cell, between the two archoplasmic 
spheres, a segmentation which grows deeper and deeper 
until, after the formation of two new daughter nuclei, the 
cell is divided into two new cells. 

The above facts are necessary in order to fully under- 
stand the union of two different cells and the subse- 
quent fertilization. As soon as the ripe ovum has left 
the ovarium (sometimes before) certain changes occur 
which are necessary conditions of fertilization. The 
germinal vesicle changes into a nuclear spindle, which 
moves towards the surface of the animal pole of the 
ovum. Accompanied by radiating phenomena of the 
protoplasm, a part of this spindle is ejected in form of a 
little ball ("directive or polar body"). In most cases a 
C* 



34 METAZOA. 

second polar body is ejected. The rest of the spindle 
assumes its former structure and moves towards the in- 
terior. It is called the "egg nucleus" or "female pro- 
nucleus." The polar bodies perish. It has been actually- 
proved that fertilization consists of a union of the nuclei 
of ovum and spermatozoon, and that only a single sperma- 
tozoon is received by the ovum. 

We find even in the lowest Metazoa a certain arrange- 
ment of the cells after fertilization or segmentation has 
been going on for some time. They form in the simplest 
Ccelenterata two strata of an epithelial character which, 
in close adhesion, constitute the wall of the tubular body, 
open at one end. In accordance with the physiological 
functions of the different cells, the lasso cells or nettling 
organs and the neuro-muscular cells form the external 
stratum; whilst the nutritive cell make up the internal 
stratum, i. e., the one nearest the intestinal canal. The 
reproductive cells lie protected in the deeper layers of 
the external stratum. These two strata, which occur in 
all Metazoa, are called Ectoderm and Entoderm. 

Thus cells or tissues of the same or of different kind 
combine to cell or tissue complexes. Such complexes 
are called organs when the cells or tissues, compos- 
ing them, together join in the performance of one or 
several functions. A primitive organ is, e. g., the ento- 
derm of a hydra, because all its cells perform the function 
of nutrition. The tentacles of the same are, however, 
more complicated organs; they serve as organs of feeling, 
as weapons and as instruments with which thejr procure 
their food. For this purpose they contain neuro-muscular 
cells and nettle organs. Their nutrition is accomplished 
by entodermal vessels which connect them with the di- 
gestive track. Thus the various elements are employed 



SEGMENTATION. 35 

in the service of one or more than one function after 
which the organs are named. Accordingly there are 
organs of special sense, of respiration, of locomotion, of 
reproduction, of nutrition, etc. Especially among the 
higher animals several organs of the same kind and 
similar function occur in one and the same body. As 
such they are parts of a system of organs: muscular sys- 
tem, vascular system, nervous system, etc. 

The question arises: How may the lowest, simplest 
Metazoa have originated? Various theories have been 
advanced in answer to this question, most prominent 
among which is the gastrcea theory, because it is founded 
upon a number of facts. It is highly probable that all 
Metazoa descend from one common ancestral foim, and 
that certain coinciding features in their embryology find 
an explanation in this common origin. The earl} 7 stages 
in the embryology of the Metazoa present a definite plan 
which is expressed in the blastula and gastrula, both re- 
capitulating ancestral forms common to all Metazoa. 

In the first embryological stages of the Metazoa there 
is forming a chief ox primary axis, whose ends are termed 
animal and vegetative pole respectively, bcause in later 
differentiations of the two primary strata the ectoderm, 
developing around the animal pole, serves the functions 
of the animal sphere (sense perception, locomotion), 
whilst the entoderm, developing around the opposite 
pole, performs the functions of vegetative life (nutrition). 
Accordingly the Metazoa present originally the uniaxil- 
lar, heteropolar structure. Frequently the primary axis 
can be recognized even in the ovum when the nucleus 
surrounded by dense formative protoplasm lies nearer 
the animal pole. It is there where the directive bodies or 
polar cells are expelled, immediately before fertilization. 



36 MKTAZOA. 

The process of egg segme?iiation with which after fer- 
tilization the embryonic development begins, is essen- 
tially a division of the ovum, steadily progressing accord- 
ing to definite laws, which result in the appearance of a 
number of still more undifferentiated cells (segmentation 
spheres, blastomeres). According to the direction which 
the planes of segmentation follow, we distinguish be- 
tween meridional (lying in the plane of the primary axis) 
and equatorial (perpendicular upon the plane of the same 
axis) furrows. In this way blastomeres arise which at 
first assume the shape of a sphere, later on that of a 
pyramid, being radially arranged around an internal 
centre of the ovum. Soon, through the separation of 
the cells, a central cavity is formed, called the segmenta- 
tion cavity or v. Baer's cavity (blastoccelum) , which con- 
stantly increases with the further development of the 
segmentations, while the blastomeres develop into a one- 
layered epithelium {blastoderm) surrounding this cavity. 
This stage in the embryology of the animal is called 
blastula or blastosphere. Even in the one-layered blasto- 
sphere, an arrangement of the parts around the primary 
axis is clearly recognized. The cells in the neighbor- 
hood of the animal pole are mostly smaller and poorer in 
food yolk, whilst those of the vegetative part are larger 
and richer in food-yolk, dividing more slowly on account 
of the impeding influence which the food-yolk exercises. 
The wall of the one;layered blastosphere presents the 
first primary organ of the metazoan body. 

From the blastula stage the gastrula is developed by 
the flattening and gradual invagination of the cell layer 
of the vegetative half, arising from the vegetative pole. 
In this process the primary segmentation cavity more and 
more disappears, and becomes often only an arrow split 



SEGMENTATION. 37 

between the two layers of the body wall produced by this 
change. The gastrula stage assumes essentially a sac- 
like form. It encloses an inner space, arisen by invagina- 
tion, which is designated gastroccelom, opening externally 
in the region of the vegetative pole by an aperture called 
the pros to ??ia or blastopore. The cells of the animal part 
of the blastosphere now constitute the ectoderm; the vege- 
tative half, which has become internal by invagination, 
forms the entoderm. In the region of the prostoma the 
ectoderm passes into the entoderm. Both layers constitute 
the two primajy organs or germinal layers. The gastrula 
stage, typical with modifications for all Metazoa, appears 
as the recapitulation of a hypothetical ancestral form 
(gastr<zd), distinguished by the development of the pri- 
mary body cavity (gastroccelom). Among the animals of 
the present time the Cnidaria have retained the essential 
structure of this ancestral form. In the higher forms 
modifications arise in the primary organs and with them 
differentiations of other organs. An intermediate la} T er, 
the mesoderm, develops between the ecto and entoderm. 
The entoderm retains its original functions of nutrition 
and digestion, forming the epithelium of the mid-gut, 
whilst the ectoderm furnishes the skin, the nervous sys- 
tem and the sense organs as well as the epithelium of the 
invagination of the fore-gut and hind-gut. 

It often happens that the segmentation, the develop- 
ment of the germinal vesicle and the process of gastrula- 
tion are modified by the presence and definite stratifica- 
tion of larger quantities of food yolk, which of course 
changes the typical development of blastula and gastrula 
stage as given above. 

Ova which are poor in food yolk (e. g., Amphioxus, 
$agitta, Echinodermata) undergo the typical segmenta- 



38 METAZOA. 

tions explained above. Here are the blastomeres of 
almost the same size, those of the vegetative pole being 
slightly thicker. This phenomenon is termed total and 
equal segmentation, because the entire mass of the ovum 
is divided into blastomeres of almost equal size. The re- 
sulting blastula stage with its large central segmentation 
cavity is designated cceloblastula or archiblastula whilst 
the gastrula following it bears the name of invagination 
or embolic gastrula. 

The ova of certain Cnidaria, especially of the Hydroids, 
undergo a somewhat different gastrulation, although the 
process may be traced back to the one just stated. It is 
called polar invasion. The cells of the vegetative pole 
separate from the blastoderm, and enter the blastoccelom, 
which is gradually filled with a dense mass of entoder- 
mal cells. The gastroccelom is only a secondary forma- 
tion, and the mouth aperture arises through a dehiscence 
of the wall. The difference between this and the typical 
gastrulation lies in the fact that the entodermal cells 
give up their epithelial connection at the very beginning 
of the invasion. 

There are cases, however, where a larger quantity of 
food yolk is deposited in the vegetative part of the ovum, 
so that the animal part appears to be considerably smaller. 
The results of segmentation, which is still total, show a 
relatively small segmentation cavity, situated eccen- 
trically near the animal pole. The wall of the germinal 
vesicle, which may still be termed cceloblastula, shows 
in this case considerable difference in its density, corre- 
sponding to the animal and vegetative pole. This type 
of segmentation is termed total, unequal segmentation, 
whilst the forms belonging to this and the preceding 
type of segmentation are together called holoblastic ova. 



SEGMENTATION. 39 

The total unequal segmentation may develop an invagi- 
nation-gastrula, but the gastrocolic lumen will be pro- 
portionately shallow on account of the small segmenta- 
tion cavity. (Lamprey.) 

In some other cases (e. g., certain annelids), ablastula 
stage is formed at the end of the total unequal segmenta- 
tion, in which the segmentation cavity is reduced to a 
minimum. A cell mass {Sterroblashda) results, in which 
a portion consisting of large entodeim elements rich in 
food yolk can be distinguished from another portion 
consisting of small ectodermal cells. The latter lies 
on the former like a cap. The gastrula stage arises by 
the growth of the edge of the ectoderm, which finally 
encloses the entoderm. This gastrula bears the name of 
epibolic gastrula (Sterrogastrula) . The gastrocceloni is 
formed later as a split in the entodermal cell mass. The 
edges of the ectodermal layer must be considered as a 
blastopore. It is accordingly filled by a so-called yolk 
plug. 

The presence of larger quantities of food yolk within 
the vegetative part of the ovum impedes the progress of 
its segmentation. In extreme cases of this kind seg- 
mentation may cease near the vegetative pole, and only 
a comparatively small portion of the ovum, consisting 
chiefly of formative yolk, may be divided into blosto- 
meres. Such ova which undergo only partial segmenta- 
tion are termed meroblastic, in contradistinction from 
those called holoblastic. A disc-like embryonic forma- 
tion arises which, corresponding to the unsegmented 
mass of the food yolk, is situated upon the animal pole. 
This type represents the extremest case of unequal seg- 
mentation, and is termed discoidal, (found among the 
Cephalopoda, the Birds, Reptiles and many Fish). 



4-0 MKTAZOA. 

A special segmentation type is developed among the 
Arthropoda. Whilst the ova so far considered are telo- 
lecithal, i. e., contain the food yolk within the vegetative 
sphere, the ova of the Arthropoda show such an equal 
distribution of the food yolk that its centre coincides with 
the centre of the ovum {centrolecithal ova). The first seg- 
mentation nucleus lies in the centre of the ovum where 
it separates into a large number of nuclei, which are 
equally distributed along the periphery of the ovum, 
there causing the formation of a layer of small blas- 
tomeres of like shape. This layer represents the blas- 
toderm, whilst the segmentation cavity of this bastula 
stage is filled with the unsegmented mass of the food 
yolk. This process of segmentation is called superficial 
segmentation. 

The modifications of development so far considered, 
appear chiefly to be conditioned by the quantity and 
mode of the distribution of the food yolk. There are 
still a few more forms to be mentioned, which indeed 
resemble, in their segmentation, the centrolecithal ova, 
but prove to be aberrant on account of the peculiar mode 
of their entodermal development. First of all is to be 
mentioned a form common among certain hydroids, 
generally termed entodermal development by (^lamination. 
It is most typical in the development of the Geryonida. 
After the formation of a coeloblastula by means of total 
and equal segmentation, an internal portion, richer in 
food yolk, is separated from a superficial portion, poor 
in food yolk. Thus, from the single layered cell-sphere 
two concentric spheres arise, the^ internal one forming 
the entoderm, the external one the ectoderm. Gastrula- 
tion does not take place through invagination, the gas- 
troccelom being produced from the segmentation cavity. 



SKGMKNTAION. 4 1 

A transition from delamination to polar overgrowth 
(so-called multi-polar invasion) has not yet been fully 
established, and the acceptance of a morula stage (with- 
out a segmentation cavity) formerly so prevalent, seems 
to be based on erroneous suppositions. 

It has been shown that the chief axis of the gastrula 
stage connects the anterior or apical pole (animal) with 
the posterior or prostoma pole. This primary axis be- 
comes in the lowest type of the Metazoa, i. e., the Pori- 
fera, Cnidaria and Ctenophora, the definite chief axis of 
the body. Hatschek, therefore, called these groups 
Protaxonia in contradistinction from the other Metazoa, 
the Heteraxonia or Bilateria. The latter name is due 
to a secondary displacement of the blastopore ; the later 
chief axis of the body can no longer be traced back to 
the primary axis. 

The stratified structure of the Metazoa becomes more 
complicated through the appearance of a layer of cells, 
which arises between ectoderm and entoderm, within the 
primary body cavity (remnant of the segmentation 
cavity) and is termed mesoderm or intermediate germinal 
layer. This name is used indiscriminately, without 
reference to the homology for all Metazoa. Among the 
Bilateria the homology of the mesoderm in all the 
groups may be accepted as probable. Some members of 
the Protaxonia show an independent development of the 
mesoderm. 

The mesoderm of the Bilateria arises generally from 
the primary entoderm which divides into the mesoderm 
and the secondary entoderm. The usual mode of devel- 
opment, namely from two different primitive mesoderm 
cells, is early observed at the prostoma of the gastrula 
stage, where their position defines the median plane 



42 MKTAZOA. 

which intersects them. They move into the space be- 
tween ectoderm and entoderm (primary body cavity), and 
form through proliferation the mesodermal streaks, two 
paired cell rows from which the organs of the mesoderm 
are developed. The other rarer mode of development, 
as found among the Chaetognatha, Brachiopoda and 
Chordonii, consists in the formation of paired, sac-like 
diverticula of the gastrocoelom, which become strictured 
and thus originate as independent ccelomsacs, the organs 
of the mesoderm. Both modes of mesodermal develop- 
ment are based upon one and the same plan, differing 
simply in regard to the separation of the mesodermal 
elements from the epithelium of the entoderm, which in 
the first case" is rapid, in the second slow and more com- 
plicated. 

In the later development of the mesoderm two types 
may be distinguished. In the one case the mesodermal 
elements loosen and completely fill the primary body 
cavity by the formation of a structure consisting of star- 
shaped migratory cells imbedded in a jelly-like sub- 
stance. This structure is called mesenchyma. (O. and 
R. Hertwig). By a separation of its cells, so-called 
lacunae may arise, which on uniting may represent an 
apparent body cavity. Such cavities are termed pseudo- 
cozlom. 

In. other cases most of the mesoderm disappears in the 
formation of paired sacs, called ccelom sacs, in whose 
walls the mesodermal cells have epithelial connection. 
The sac cavities represent the genuine body cavity or 
ccelom. The paired ccelom sacs completely enclose the 
intestinal canal, and form in the median line above and 
below it the so-called mesenteries. The body cavity 
separates the mesoderm in two layers. The external 



SEGMENTATION. 



43 



layer, adjoining the ectoderm is called the somatic layer, 
the internal layer adjoining the entoderm is called 
splanchnic layei . 

In some animals (Plathelminthes) the mesoderm fur- 
nishes, besides, or aside from its specific organs (of repro- 
duction and excretion) only mesenchyma. Bat in the 
majority of the Bilateria the formation of mesenchyma 
and ccelom are contemporaneous and, as it were, competi- 
tive, so that in one case (Annelids, Sagitta, Pharonis), 
the latter, in the other (Mollusca, Arthropoda) the 
former predominate. 

From the mesoderm of the Bilateria are derived the 
muscles, genital organs, nephridia, connective tissue 
and fatty tissue. 

The following table presents the different modes of 
segmentation. (See table of Hatschek's classification.) 

table:. 



Equal. 


Porifera. Scolecida. 
Cnidaria. 


Annelida. Echino- 
Crustacea. dermata 
Mollusca. 


Tunica la. 

Amphioxus 
(among Mamma- 
lia in secondary 
form.) 


Unequal. Porifera. Scolecida. 
Cnidaria. 
Ctenophora. 


A n nelida. Echino- 
Crustacea. dermata. 

Mollusca. 


PetKomyzon tides 
(Lampreys). 
Ganoidei 
Amphibia. 


Discoidal. 


Cephalopoda. 


Selachii. 
Telcostei. 
Reptilia. 
A ves. 
Monotremata. 


Superficial. 




Arthropoda. 





Note : The italic type shows the predominant segmentation. 



The successive division of the egg cell results in the 



44 METAZOA. 

formation of groups of cells which are united in their 
functions to so-called tissues. Tissues of one or more 
than one kind form the organs of the body. The higher 
the animal the more specialized becomes the division of 
labor, i. e., the physiological functions of the various 
tissues. This leads us to the consideration of Metazoan 
histology. Here, two factors are to be emphasized : (i) 
the lamina of the cells ; (2) their structure and their 
products. 

The epithelium is the oldest and most important 
giound-form of the Metazoan tissue. It consists of cell 
masses arranged in a plane, and is already met within the 
blastoderm of the blastula, where it is a simple layer, 
and therefore called one-layered. It presents two planes; 
an external open plane and a basal plane turned towards 
the blastoccelom. Accordingly, each epithelial cell has 
two poles, a free pole and a basal pole. Both planes pos- 
sess in all cases opposite characteristics. One layered 
epithelia are classified according to their form into cylin- 
drical, cubic, and pavement epithelium. Stratified (of several 
layers), epithelia (in skin of vertebrates), are -not of the 
same formation throughout, (flat above, round below), 
it is, however, of rare occurrence that a complete separa- 
tion into lamina (delamination) takes place. 

Tissues, originally consisting of connected epithelial 
tissues, often change in mode of layering and surface 
formation. They are called epitheliogenous. 

Individual basal cells of the epithelium may enter the 
blastoccelom in an amoeboid form, and thus be degraded 
to apolar cells. (Mesenchyma cells). They may differ- 
entiate into tissues (mesenchymal tissue) and develop 
again epithelia of internal cavities. Such formations 
are termed pseudo-epithelia or epitheloids, 



HISTOLOGY. 45 

All histological differentiations proceed from the cell 
plasma, the neuclus retaining its typical character. Di- 
rect plasma differentiations manifest themselves in certain 
plasma functions (contractility, irritability) in a high 
degree (muscular and nervous substance). Other differ- 
entiations are simply products of. the plasma. The for- 
mer are called autoplasmatic, the latter apoplasmatic struc- 
tures, 

i . Autoplasmatic structures. External plasmoid cellfo? - 
mation. Pseudopodia. They have the same functions 
as those of the Protozoa. Amoeboid epithelial cells occur 
in embryonic forms for the purpose of yolk reception ; in 
the ectodermal epithelium of fully developed sponges, 
and in the entodermal epithelium of all the various 
lower and higher Metazoa for purposes of nourishment. 
Amoeboid mesenchyma cells retain their original form in 
the embryonic mesenchyma cell and in the free comiective 
^//(wandering cell). Similar to it are the a?nceboid pigment 
cells, where pseudopodial change of form brings about 
change of color; finally amoeboid (or white) blood cor- 
puscles. Vibratile, hair like processes are present on the 
free surface of all epithelial tissues (except in certain 
protonephridial canals). There may be either one on the 
surface of each cell, called flagellum, or many, called 
cilia, or a compound flagellum or compound cilia {ciliated 
plates). The Arthropoda, Nematodes (and Acantocephali) 
are without these processes. They perform functions of 
locomotion (small ciliated swimming Metazoa and larval 
forms), of feeding (around the mouth of Rotatoria), of 
digestive respiration {ciliated epithelium of intestinal canal) ; 
of pulmonary respiration (ciliated epithelia of gills). 

Epithelial cells which possess at their free end sensory 
hairlike processes and are at their basal end connected with 



46 M3TAZ0A. 

nerves are termed sensory cells. These sensory processes 
are either sensory hairs (of hearing or touch), or bristles 
or rods (eye). The last named may assume various modi- 
fications and even be found in the interior of the plasma 
of the visual cells. The bristlelike processes on the free 
surface of the epithelial thread cells of Cnidaria are called 
cnidocils. They all have sensory functions. 

Internal plasmoid cell formation. Muscular fibrils and 
tissue. Structural. The muscular fibril arising as the 
contractile element within the plasma of the muscular 
cells {myoblasts) is a threadlike, pellucid substance of 
varying lengths, either pointed or blunt (rarely forked or 
netted) at the ends, which, arrayed in bundles or in strata, 
constitutes one or several cells. In smooth fibrils the con- 
tractile substance is continuous, in cross-striped fibrils it 
is divided into sarcous elements, which are separated by 
non-contractile disks. The difference between the two 
kinds of fibrils shows a difference of function. Muscles 
frequently and voluntarily used, also those of heart, pre- 
sent a striated appearance and are mostly found among 
the Vertebrates. 

The myoblasts are either epithelial muscle cells, with a 
muscular process at their bases, and possessed of various 
functions, or muscle fibres, situated in the deeper parts of 
the body, and having the appearance of a piece of cord. 
They consist of an external membrane {s arco lemma) , of 
fibrils and a small portion of unchanged plasma {muscle 
corpuscles). Genetically one kind of muscle fibre is more 
closely related to the epithelial muscle, the other to the 
mesenchyma. There are, therefore, three kinds of mus- 
cular tissue: Epithelial muscles, epitheliogenous muscles, 
and mesenchymal muscles. Epithelial muscles belong ex- 
clusively to the Cnidaria. They arise on' the basal sur- 



HISTOLOGY. 47 

face of the epithelial cells, running parallel to one 
another in definite directions. A massi?ig of fibrils, 
(rarely by stratification), is mostly due to a longitudinal 
folding of a single fibrinous layer which thus may become 
very high. The epitheliogenous muscles of the Cnidaria 
arise through a constriction of the fibrinous folds which 
form with their myoblasts subepithelial cord-shaped 
structures. Delaminatio?i between surface stratum and 
myoblastic stratum occurs but rarely. Among higher 
animals a large portion of muscular tissue arises from 
the epithelium of the ccelom, and its mode of separation 
shows innumerable degrees of gradual variation. 

The mesenchy??tal muscle fibres originate phylogeneti- 
cally in the gelatinous substance which fills the blasto- 
ccelom of the lowest Metazoa. Primarily they exist as 
single fibres. Tissue masses are secondary formations. 
They are present, in some Porifera, in the Ctenophora ; 
in the lowest worms exclusively ; hardly in higher 
worms; very prominently in the Molluscs, and some- 
what in the intestinal muscles of Bchinodermata, and 
the Vertebrates. The movements of the smooth muscle 
fibres are slow, but much more intense than the quick 
movements of the striped muscles. 

The essential elements of the nervous system are gan- 
glion cells and nerve fibres. It appears in the lowest form 
as a ganglion-plexus spreading like a net within and 
throughout the epithelia (Actinia); whilst in higher 
forms numerous ganglion cells unite to so called ganglia, 
whence the nerve fibres branch out in bundles, as nerves, 
becoming thinner and thinner towards the periphery, 
until they consist of only one fibre, which may again 
join a ganglion-plexus. The structural element of nervous 
tissue is the nerve fibril (primitive fibril). It is a very 



48 M3TAZOA. 

thin, simple thread, with characteristic little knots. A 
number of them surround concentrically the nucleus of 
the ganglion cell, continue radially into the contents of 
the nerve fibre (which in its peripheral anastomosis sep- 
arates into fibrils), and may finally penetrate the interior 
of the peripheral cells (e. g., the sensory cell or muscle 
fibre.) 

Ganglion cells are round, have a large nucleus within 
rich fibrinous protoplasm, and possess one or more pro- 
cesses (unipolar 1 bipolar, multipolar, ganglion cells). 
These processes may either continue as a nerve {nerve 
or axis cylinder process) or form an anastomosing net- 
work (plasma process). The nerve fiber consists essen- 
tially of the axis-cylinder which is of varying diameter 
and of a slimy character. In its simplest form it is 
without a surrounding fatty marrow (non-medullated), 
and as such it is present in all Invertebrates and some 
lower Vertebrates, as well as in the sympathetic and 
olfactory nerves of the Vertebrates. The axis cylinder 
possessing a double contour (medullated) is present in all 
Vertebrates. Besides, in the peripheral nerves of many 
Vertebrates and the Arthropoda a nucleated sheath 
(Schwann' s) surrounds the axis cylinder, whilst in the 
central nervous system it is enveloped by a continuous 
tissue of fibrous cells (neuroglia or glia cells). The same 
may be said of the envelopment of the axis cylinders of 
peripheral nerves among Annulata and Mollusca. Axis 
cylinders of peripheral nerves are perhaps only length- 
ened processes of central ganglionic cells. 

Peripheral nerve- ends generally combine with cells of 
widely varying functions, among which may be men- 
tioned sensory and muscular cells. The former join the 
nerve at their basal pole; the latter along the side of the 



HISTOLOGY. 49 

fibre, where trie nerve ends in a plate (striped muscle fibre 
of Vertebrates and Arthropoda). The same can be said 
of the muscle fibre of the Vertebrates and mesenchymal 
muscle of the Invertebrates. (The smooth muscles of 
the Vertebrates are enveloped in a fine nervous network.) 

The stratification of the nervous tissue is either epithe- 
lial or epitheliogenous. 

The epithelial nervous system of the Cnidaria is of spe- 
cial importance for nervous phylogeny. In the ectoder- 
mal epithelium of the Actinia are present the following 
sensory and nervous elements: Sensory cells on the free 
surface, ganglion cells in the middle and nerve masses 
near the base, (muscular cells beneath if such are pre- 
sent). The nerves form a network with the sensory and 
ganglion cells. 

The Hydromedusae and Scyphomedusae present the 
beginning of a central nervous system. This always de- 
velops zuhenever sensory organs arise. The formation of 
such organs depends upon the stratified accumulation of 
sensor}' cells and nervous elements, as mentioned above; 
all other cells, except supporting cells, are absent. Such 
a specialized epithelium is termed a nervous sense-epithe- 
lium. The Scyphomedusae possess eight ganglionic cen- 
tres, radially distributed, in connection with eight groups 
of sensory organs. Among the Hydromedusae the cen- 
tral nervous system assumes the form of a ganglionic 
ring which stands in connection with sensory organs, the 
body epithelium being interlaced with a fine nerve 
plexus. The nervous system of the higher animals (He- 
teraxo?iia), is always central, but may retain its epithe- 
lial position. Not only the central nervous system and 
sensory organs but also the beginnings of the peripheral 
nerves of many Annulata (Sagitta), Brachiopoda, Echi- 



50 MKTAZOA. 

nodermata, and of Balanoglossus, develop from and lie 
within the ectodermal epithelium. The sensory epithelia 
of the sensory organs consist largely of sensory cells, 
ganglion cells, nerve fibres and supporting cells ; the 
nerve epithelia of the central system of ganglion cells, 
nerve fibres and supporting cells ; the nerve epithelia of 
the peripheral system of nerve fibres and supporting cells. 
Stratification is the same as among the Cnidaria. 

The nervous system of the most highly developed ani- 
mal gives up its original epithelial position and seeks a 
more protected situation within the body. Thus its 
anatomical separation becomes perfect. Those groups 
of animals which show all the transitions from the epi- 
thelial to the epitheliogenous nervous system, often within 
the same body, are of special interest, because we ob- 
serve, that anatomically and histologically the epithelial 
supporting tissue gradually develops the glia substances of 
the nervous system. 

The separation of the central nervous systems occurs 
either by delami?iation or by invagination. The first 
mode is predominant among the Invertebrates. The 
ganglionic and nerve layer, together with the deeper layer 
of supporting cells, separates from the surface layer. The 
other mode is predominant among the Vertebrates. A 
part of the ectoderm along the dorsal surface of the 
embryo caves in towards the interior of the body, sepa- 
rates and forms an epithelial free tube {medulla), becoming 
through histological differentiation a nerve epithelium. 
The stratification is here the same as among the Cnidaria. 
(Amphioxus a good example.) 

The sensory epithelia either lie in the same plane with 
the other ectodermal epithelium or form deeper invagina- 
tions. Often they separate in form of bladders (eye blad- 



HISTOLOGY. 5 1 

ders, auditory bladders, touch bodies). There are two 
kinds of sensory epithelia: stratzfed epithelia, which pos- 
sess beneath the sensory cells a ganglionic layer and a 
basal nerve layer; simple epithelia, consisting only of 
sensory cells, to whose basal termination nerves are at- 
tached. Often a peripheral or sensory ganglion lies near 
the simple epithelium. A comparison between the de- 
velopment of the vertebrate retina and that of a cephalo- 
pode furnishes an interesting illustration for the fixed 
arrangement of stratification. 

The development of peripheral nerves is only traceable 
among the Vertebrates. They arise as outgrowths of the 
central nervous system, uniting secondarily with their 
terminal organs. 

The fiinctioiis of the nervous tissues are complex, begin- 
ning as crude irritation in the simple cell plasma of the 
lowest animals, and reaching their highest differentia- 
tions with the complete division of labor in the minutely 
organized nervous system of man. Sense organs, nervous 
organs and locomotory organs stand in necessary mutual 
relation to one another, just as sensation and the result- 
ing action bear a necessary relation to each other. Spe- 
cialization is carried so far that each form of sensation 
has its special apparatus of transmittance to the central 
nervous system. In many lower animals the ectodermal 
epithelium performs all the functions of sensation 
(touch, light, heat, chemical irritation) and is, there- 
fore, to be considered as a primitive sensory organ. 
From this the special organs developed. First, the 
integumental se?ise organ. They are either isolated sen- 
sory cells (Cnidaria), or sensory buddings, groups of 
sensory cells, often surrounded by a concentric zone 
of supporting cells, connected together by a nerve 



52 METAZOA. 

fiber (higher Invertebrates). The epithelium of the Ar- 
thropoda secretes a chitinous cuticle with hollow bristle 
and hairlike processes; special cells of the matrix send 
thin plasma threads into these processes, and per- 
form the functions of cells of touch, since they are con- 
nected with a nerve at their base. Vertebrates living in 
water show the same integumental sensory organs, ap- 
pearing either as solitary cells, or as cell buds, as free 
nerve terminations or as so-called lateral line organs of 
the fish. Organs of touch lying in the cutis appear also 
in the form of peculiar sensory cells among the terrestrial 
Vertebrates. 

Organs of taste and of smell perform the function of 
testing the food preparatory to the various modes of 
metabolic changes. In the lowest animals the sensory 
cells of taste are like those of touch, and are only dis- 
tinguished by their position in or around the mouth 
opening. The organs of smell of the Invertebrates have 
the form of papillae, with vibrating epithelium, contain- 
ing vibratory and sensory cells. It is situated near 
nerve centres or higher sensory organs The Crustacea 
perform the functions of taste and of smell by means of 
so-called olfactory hairs situated along the first pair 
(rarely also on second) of the antennae, and filled with 
plasma, which, through a round cell (sensory cell?) 
connects with a nerve fiber. The olfactory organs of the 
insects are situated along the antennae, their organs of 
taste along the palpi or mouth cavity. Both arise from 
the chitinous trichome. The sensory elements are cone- 
shaped and lie, single or in groups, either free on the 
surface of the chitinous cover or in a papilla. The 
membranous canals of the antenna are modifications of 
these organs. The olfactory organs of the Vertebrates 



HISTOLOGY. 53 

resemble originally the structure of paired ciliated 
papillae lying in the anterior end of the head. After- 
wards they move interiorly ; nose cavities arise without 
sensory epithelia. They open in the mouth cavity, and 
serve as air passages. Their olfactory epithelium con- 
sists of high ciliated cells and sensory cells with free 
sensory hairs. 

The sensory cells of the auditory organs resemble the 
primitive cells of touch in form and function. They 
always require an auxiliary apparatus to receive the 
sound waves. In certain Hydromedusae tentacle for- 
mation are changed into auditory vesicles whose ento- 
dermal axillary cells carry otoliths and whose octodermal 
sensory cells carry sensory hairs, which are set in 
motion by the sound waves. The deeper the position of 
these vesicles the higher is their function. The auditory 
vesicles of the Leptomedusae are of independent origin 
arising radially along the edge of the disk as numerous 
invaginations of the ectoderm. The otoliths lie in the 
ectodermal, cells and communicate through vibrations 
with the neighboring sensory cells. 

The auditory organs of Turbellaria, Annulata and 
Mollusca (exclusively) are vesicles derived from the 
ectoderm containing one or more calcareous secretion 
products (otoliths) suspended by ciliated cells. The 
wall of the vesicle contains sensory cells which send 
hairs into the lumen. 

In spiders and mites the auditory organs are absent. 
In the higher Crustacea they are at the base of the first 
antennae, consisting of epithelial sacs opening externally. 
They are internally covered by a continuation of the 
external chitinous epithelium, upon which rows of very 
active auditory bristles are situated; grains of sand enter 
from the outside and act as otoliths. 



54 MBTAZOA. 

Auditory organs are peculiar and numerous in the 
insects. They are groups of cells which are stretched 
like the cords of a musical instrument through the body 
cavities, fastened by a ligament at opposite ends of the 
integument, to one of which the nerve is attached. This 
group of cells contains ganglion cells and sensory cells, 
the latter containing peculiar bodies called auditory 
rods. In musical insects a trachea forms a tympanum, 
lying directly under the thinned chitinous membrane, 
thus perfecting auditory functions. These organs are 
situated in different insects at different places. 

The auditory organ of the vertebrates undergoes the 
simple vesicular development, in which condition it often 
contains otoliths, and is capable of performing functions 
(in embryonic teleosts). This vesicle develops into a 
labyrinth with peculiar characteristics. In it auxiliary 
apparatuses arise which conduct and magnify the sound 
(external ear, tympanum, auditory ossicles). The phys- 
iological function in distinguishing sound differentia- 
tions depends upon the transmittance of these sounds 
upon the auditory cells by means of auxiliary appara- 
tuses (fibres of-membrana basilaris). 

The simplest eyes consist of a small number of sensory 
cells (even of only one) which on one side are covered 
by a goblet-shaped, opaque mass of pigment, so that 
light can penetrate the sensory cells only in one direc- 
tion {visual axis), which is observed by the animal. 
Such eyes are called euthyscopic (or directing) eyes. 

The Cnidaria possess such pigmented spots of sense- 
epithelia, in which each sensory cell is surrounded by 
supporting cells. They are called ocelli. The eye of the 
Turbellaria and Rotatoria possesses in addition a light 
refracting apparatus or lense, which however only serve§ 



HISTOLOGY. 55 

the function of intensifying the light-effect. The tran- 
sition from the euth}~scopic to eidoscopic eyes, i. e., eyes 
which perceive a picture, generally takes place by an 
aggregation of the former. A large number of luminous 
elements, i. e., sensory cells with their specific termina- 
tions, which are connected with separate nerve elements, 
are necessary in order to see a picture. The sensory 
cells compose a sense epithelium, which is here termed 
retina. The larger the number of the luminous elements 
of the retina, the more detailed is the perceived picture. 
This differentiation of separate sensory cells with sepa- 
rate nerves for the perception of separate sensations, is 
called the nervous isolation of the perceiving elements. 
A second necessity is the optic isolation of the perceiving 
elements, which is to prevent a general diffusion of light 
upon the retina by admitting only the rays proceeding 
from single definite objects. This is done in various 
ways. We therefore distinguish different types of eyes. 
The most perfect eyes are the camera eyes, which possess 
special light-refracting apparatuses for optic isolation. 
The eye of the vertebrates belongs to this type. The 
human eyeball consists of a ha?'d cuticle {sclera), an 
external firm connective tissue layer, which passes ante- 
riorly into the cornea, a convex transparent horny cuti- 
cle, with an external transparent epithelial layer. From 
the interior of the sclera arises the chorioidea, a second 
connective tissue layer, rich in blood vessels and dark 
pigment, continuing anteriorly as a contractile mem- 
brane, the iris, with an open circular centre and a thick- 
ened muscular ring, the ciliary body as its root. The 
next interior layers are the black pigment epithelium 
and the bright retina. Not far from the ciliary body, at 
a line called ora serrata, the nervous character of the 



56 M^TAZOA. 

retina ceases, and becomes a pigmented layer, which, 
together with the pigment epithelium, forms a double 
layer, and extends to the free edge of the posterior sur- 
face of the iris. The anterior eye chamber between cornea 
and iris, is filled with aqueous humor, whilst the posterior 
eye chamber, behind the iris, contains the lens, onto- 
genetically derived from the external epithelium. The 
lens capsule is fastened by connective tissue fibres 
{zonula Zinnii) to the ciliary bodies. Behind the lens 
the cavity of the eyeball is filled with the corpus vitreum, 
a less pellucid, transparent connective tissue substance. 
The optic nerve, entering the eyeball not quite parallel 
with the optic axis, penetrates the retina and continues 
in its nervous layer. Then follow the two ganglionic 
layers, the optic cells, and finally the rods which are 
turned towards the pigment epithelium. 

The picture is projected upon the retina (reversed as in 
a camera), after the rays of light have been deflected 
three times; in aquatic animals the deflection of the rays 
in passing from the water to the cornea is but slight, 
and the work is largely done by a strongly convex lense, 
following the laws of optics. The eye is adjusted or ac- 
commodated to different distances by changing the form 
of the lens by means of the ciliary muscles. 

The eyes of many Invertebrates (especially Cephalo- 
poda) are of similar physiological structure as those of 
the Vertebrates, but they are morphologically different; 
the rods of the retina are turned towards the eye pupil. 
Eyes of insects and spiders, without facilities for accom- 
modation, are probably only for definite distances. 

The compound eyes of the Crustacea and Insecta are 
termed convex musivian eyes, a second physiological type 
of the eidoscopic eye. The perceiving elements of the 



HISTOLOGY. 57 

retinae are here groups of cells, called reti?iulcz. In the 
centre of the ret inula is a rodlike structure, the rhabdom, 
consisting of as many rhabdomens as there are cells, and 
surrounded by a pigment mantle. The anterior part of 
the pigment tube is filled with the so-called crystalline 
cone. The external transparent cuticular chitinous cover 
of the eye, called cornea, is divided into very regular 
hexagonal facets, often thickened and thus representing 
each a lens {corneal lens). Only a very small cone of 
rays reach each retinula, which lessens the strength of 
the light, showing the eye to be inferior to the camera 
eye. No apparatus for accommodation is needed ; the 
animals can see in the air as well as in water. 

The co7icave-musivia7i eye of Atrhropoda and Mollusca 
is based upon the same optic principle as the preceding. 
The axillary rays of light proceeding from the object to 
the elements of the retina intersect in front of the latter, 
and throw apparently a reverse picture upon its surface. 
Only one lens suffices for all retina elements, but only 
the central rays are of use here, whilst those of the cir- 
cumference are absorbed by the pigment. Wherever 
the lense closely approaches the retina, i. e., where the 
formation of a picture is impossible, and where the retina 
is pigmented throughout, there we have a concave- 
musivian eye. 

There are transitions from the lowest to the highest 
eidoscopic eye, based upon the distance of the lens 
from the retina and its proper correction. 

Morphologically the eyes are divided into cup-shaped, 
vesicular, inversely vesicular and compound eyes. The 
first form a cup-shaped depression of the epithelium. 
The retina continues laterally into the epithelial cells. 
In their simplest form they are concave-musivian, when 
D* 



58 METAZOA. 

a cuticular or secretory lens is present, and in proper 
position they are camera eyes. The second form arises 
when the optic depressions or papillae form a vesicle and 
separate completely from the epithelium. The free sur- 
face of the epithelium becomes the interior of the vesicle 
and forms posteriorly the retina, anteriorly the internal 
pellucida, whilst the basal surface of the epithelium be- 
comes the exterior of the vesicle, producing posteriorly 
the nerve, anteriorly the external pellucida. A lens is 
always present, undergoing modifications in certain 
cases. In their simplest form they are concave-musivian 
eyes, but go through all the higher gradations. The 
third form develops from a vesicular structure called the 
primary eye vesicle, which in the Vertebrates arises by 
constriction from the medulla tube — in Invertebrates by 
direct invagination of the external epithelium. The 
anterior half of this vesicle invaginates, forming, with 
the posterior half, a double-walled cup called, secondary 
eye cup. Its anterior part furnishes the retina; its 
posterior, a pigment epithelium; here also the rods 
arise, whilst the nerve develops from the opposite sur- 
face of the retina. The lens is always a cellular structure 
which is independent outside of the primary eye vesicle. 
In the fourth form (a. facetted eyes of Arthropoda) every 
element of the eye corresponds to the two-layered cup 
eye of the Myriopoda, an aggregation forming the com- 
pound character, which aids co-operation but lowers in- 
dividual ability. Embryologically they arise from a 
thickening of the epithelium, in which already the om- 
matea (retinula with crystaline body) separated by sheath 
cells (pigment) may be recognized. The ommateum 
itself consists of the crystalline cells (upper layer) sur- 
rounded by the chief pigment cells, and of the retinula 



HISTOLOGY. 59 

cells (lower stratum) to which a nerve is attached. The 
ommatea become high and narrow and the cuticular 
cornea arises, often enlarging to a lense above each eye. 
According to the differentiation of the crystal cells there 
are: (i) acone eyes (crystal cells representing crystalline 
body); (2) eucone eyes (cells secrete a crystalline cone;; 
pseudocone eyes (transparent pseudocone arises before the 
crystal cells). 

The compound eyes of Crustacea are phylogenetically 
different from those of the Insecta, but resemble them 
very decidedly, having cornea, crystalline cells and 
cones. 

b. Middle cup eyes of Scorpion show composition of 
retinulae. The simple cup eyes of the Myriopoda consti- 
tute the origin for several different embryological de- 
velopments among the Tracheata. 

II Class. Apoplasmatic Structures. Internal cell 
formations. Water vacuoles may arise both in epithelial 
and mesenchyma cells. They become especially conspic- 
uous in the endodermal cells of the Cnidaria and in the 
cells of certain supporting tissues (connective tissues), 
which are greatly extended by them; such tissues are the 
endodernal tentacle axes of the Hydroid polyps and of the 
medusae as well as the chorda dorsalis of the Vertebrates 
and the saclike connective tissue of Platheln^nthes and 
Mollusca. Fat globules are present, especially in the en- 
dermal cells of the lower animals, forming numerous 
little drops. The so-called fatty tissue of Insects and 
Vertebrates contain fat cells in which fat globules are 
deposited. Pigment granules of various kinds are sec- 
ondary formations in the plasma of certain cells, where 
they produce district colorations. They are of special 
interest in the eye, as mentioned above, and in their 



60 MKTAZOA. 

amoeboid form, as found beneath the body epithelium. 
The power of changing the color is due either to an in- 
dependent contractility of such cells or to muscle fibres 
which are attached to them (as in Cephalopoda). Intra- 
cellular skeleton formations arise most frequently in the 
mesenchyma cells of the sponges (resembling those of 
Forminifera and Radiolaria) as calcareous needles and 
silicious bodies, which, however, during growth break 
through the cells. We can, therefore, hardly distinguish 
between inter- and intra-ceWnlar formations. The cal- 
careous skeletons of the Kchinodermata may be of 
similar origin. In this connection, the epidermal for- 
mations of the Arthropoda (chitin) and the higheJ 
Vertebrates (nails, hoofs, claws) should also be men- 
tioned. Nematocysts are peculiar, complicated, envel- 
opes of certain cells ; they are thick-walled papillae 
which contain a fluid and a nettling thread spirally rolled 
up, and are especially found in the epithelia of the Cni- 
daria, Turbellaria and iEolida. 

Ceix Secretion: Glandular tissues. Secretions are 
generally excluded from the cell and also from the body. 
Such cells are called glandular cells, and are epithelial 
in their character. Unicellular glands lie either within 
the other epithelial cells {goblet cells ; granular cells, etc.), 
or beneath them when they are differentiated into gland 
and duct. Multicellular glands arise when all the cells 
in a certain part of the epithelium become glandular 
and undergo invagination, which differentiates into 
gland and non-secretory duct. They are either tubulous 
or acinous. Secretions may serve definite purposes 
(digestive glands, poisonous glands, protecting envel- 
opes and shells of Mollusca and of Brachiopoda, ecto- 
cysts of Bryozoa). 



HISTOLOGY. 6 1 

External cell secretio?is arise along the periphery of the 
cells, originating the real connective substance. In its 
most primitive form it is the cell membrane; it develops 
also the yolk membrane of ova and the sarcolemma of 
muscle fibres. Similar secretions appear (i) in epithelial 
tissues as a cement between the cells, sometimes separat- 
ing them and forming isolated cells ; or as a basal mem- 
brane, a thin but very fine secretion under the epithelia, 
sometimes of fibrous structure ; or as cuticula, a mem- 
brane lying on the free surface of the epithelia. In 
some groups (Sponges and Vertebrates) it is not devel- 
oped; in others (Nematodes and Arthropoda) it becomes 
stratified and influences most profoundly the whole or- 
ganization. Endodermal epithelia form a so-called rod 
cuticula, a thick, superficial layer perforated by fine 
canals. Such secretions appear (2) as the fimdamental 
substance of the different kinds of connective tissue. A 
cement substance is likewise secreted by mesenchymous 
cells forming the so-called genuine connective substances, 
•especially of the Vertebrates. The different kinds of 
connective tissue show different structures of the fun- 
damental substance. Hyaline cartilage has a homo- 
geneous, elastic, firm, fundamental substance, round 
cartilage cells without blood vessels. Fibrous cartilage 
shows fibrous structures in the fundamental substance. 
Fibrilla? connective tissue is soft, but tough, its cells are 
spindle-shaped; elastic tissue is nearly related to it. Bony 
tissue is fibrillar but very firm and hard through the 
deposition of calcareous matter; it is characterized by its 
stratification. We must therefore distinguish between 
incrus ted cartilage and genuine bone tissues. The former 
is cartilaginous tissue, hardened by the deposition Qf 
calcareous salts. # 



62 MKTAZOA. 

The gelatinous tissue of the Cnidaria found between 
the two primary layers, is free from cells in the Hydro- 
medusae whilst the Scyphomedusae show mesenchymous 
cells and fibrous structures. 

Fluid substances of the; body are contained in the 
primary body cavity, in the coelom, and in the blood 
system; they differ morphologically, consequently also the 
corpuscles derived from them. The fluid of the first two 
cavities is called lymph, that of the third blood. It con- 
sists of water, in which albumen and salts are dissolved. 
The water has to permeate first the surrounding tissues 
by which it is -chemically affected. Free cells are also 
present in the fluids, which are either white blood or 
lymph corpuscles or red blood corpuscles, whose color- 
ing matter, hcemaglobin, serves the purpose of oxidation 
and deoxidation. They are generally found in Verte- 
brates (except Amphioxus) and a few Invertebrates 
(worms). 

The functions of the Metazoan body may phylogeneti- 
cally be divided into those of the ectoderm and those of 
the entoderm. Metabolism includes nutrition, respi- 
ration, excretion and circulation. The first process of 
nutrition is assimilation, which is chiefly carried on 
by the entodermal epithelial cells of the digestive canal. 
They perform the functions of the digestive activity and 
of the resorbing activity. In the sponges digestion 
is carried on by the whole body, and is termed gen- 
eral intracellular digestion; in the Cnidaria and Tur- 
bellarla it manifests itself as {special entodermal) intra- 
cellular digestion, i. e., solid food particles are taken 
up and liquified by the entodermal cells. In the 
higher animals it is called extra- cellular digestion, be- 
cause the phenomenon occurs within the digestive canal, 



HISTOLOGY. 63 

whilst the epithelial cells resorb the food (already liqui- 
fied). The division of labor is carried still further when 
the digestive apparatus is differentiated into stomach, 
glands, anus, etc. The distribution of food in the body is 
carried on in various ways in the different groups of ani- 
mals; in the Sponges by the amoeboid activity of the 
body cells, in Cnidaria by a multifarious branching of the 
digestive canal, until in the higher animal a special dif- 
ferentiation of vascular systems (blood-lymph) is called 
into assistance. The reception, distribution and elimina- 
tion of gases (oxygen and carbon dioxide) are carried on 
by the functions of respiration, either by the whole ecto- 
dermal layer of the body (in the lowest animals) or by 
an evagination (gills) or invagination (lungs) of the 
body wall, aided by the system of circulation. The 
tracheae and tracheal gills of the Insects deserve special 
explanation. The process of excretion or the separation of 
urine and related substances calls into cooperation all 
the tissues of the body. Of great importance are the 
various methods by which these nitrogenous excretions 
are collected into special organs and thence carried out 
of the body. In the lowest forms there are no special 
organs of excretion present; in Cnidaria and Ctenophora 
numerous unicellular glands of ectoderm or entoderm 
perform excretory functions, whilst in the Hydromedusae 
openings along the peripheral branches of the digestive 
apparatus have been observed. In the lower worms a 
protonephridium (or water vascular system) is present, a 
paired tubular formation, either simple or branched, in 
which strong ciliation is developed. In the Annulata, 
Arthropoda, Mollusca and Molluscoidea the protoneph- 
ridium is only present in the embryo; it is replaced in 
the adult animal by the metanephridium, an apparatus 



64 MKTAZOA. 

consisting of glandular ciliated tubes, connected by an 
internal opening with the ccelom cavity, and opening ex- 
ternally on the surface of the body. It is differentiated 
into a ciliated funnel; a canal, covered with vibratory 
threads and a muscular terminating bladder. The first 
receives the fluid from the ccelom cavity, the second furn- 
ishes the excretory substances, and the third serves as a 
reservoir. In the Vertebrates the apparatus is termed 
kidneys. It consists of segmental canals, similar to those 
of the segmental organs of the Annulata and of kidney 
ducts (Wolffian duct), probably derived from the exter- 
nal epithelium and running along the body, near, but 
outside of the splanchnoccelom. The differentiations are 
varying. 

A system of circulation is found only in those animals 
which have a secondary body cavity (ccelom cavity). It 
is, of course, wanting even among some of them. The 
walls of the blood vessels consist of two layers, an inter- 
nal epithelial layer and an external muscular layer; 
numerous differentiations are possible. The blood sys- 
tem is either closed, i. e., it does not communicate 
directly with the ccelom* cavity (Annulata), or it is open, 
when it merges into other body cavities, as we find it 
among the Mollusca. The development of a heart is 
somewhat complicated, beginning as a mere pulsating 
differentiation in the simple vessel, and reaching its 
highest form in the perfect double vessel of the Verte- 
brates. The circulatory system of the Vertebrates is 
divided into three distinctive parts: the arteries, which 
conduct the blood from the heart to the organs; the cap- 
illaries, which form the fine anastomosing net within the 
organs; and the veins, which collect the blood and con- 
duct it back to the heart. The relation of this system 



HISTOLOGY. 65 

to that of respiration is of the highest physiological im- 
portance. In the Invertebrates there is a so-called body 
heart; the blood empties from the heart into the body, 
thence into the gills and back to the heart. The Cephal- 
opoda have special gill hearts, while in the Annulata the 
flow is direct from the body into the gills. Further dif- 
ferentiation results in the gill heart of the fish, then in 
the incomplete double circulation of the Amphibia (where 
the circulation of the lungs is separate from that of the 
heart, but the blood of both mixes), reaching finally 
the climax in the complete double circulation of the high- 
est Vertebrates, where a body heart and a lung heart are 
distinctly separated. The blood flows from the former 
into the body, thence into the lung heart, through this 
into the lungs and back to the body heart. The form 
of the heart gradually changes from the periphery to- 
wards the centre, culminating in the four-chambered 
organ of the highest animals. 

The fundamental phenomena of loconiotio?i are the same 
in the Metazoa as in the Protozoa : Amoeboid locomotion, 
plasma contractility, ciliary locomotion and muscular con- 
tractility. They may all occur at the same time in the 
same bod}?-, but the last one is of greatest importance. 
Amoeboid locomotion serves mostly the purpose of dis- 
tributing the food throughout the body (Amoeboid 
migratory cells, white blood corpuscles, lymph cells); 
it also brings about the removal of injurious bodies, and 
plays an important part in embryonic development. 
Plasma contractility is similar to the first, and occurs in 
all tissue- cells which contain protoplasm. Ciliary 
motion is prevalent among the lower Metazoa, because 
only small organisms can be moved by it. Muscular 
contractility takes place among all Metazoa, and depends 
upon the nervous system. 



66 MKTAZOA. 

Phylogenetic and ontogenetic development of the tissues. 
Simple tissues consist of cells which are alike in compo- 
sition and function. A division of labor produces mixed 
tissues, whose cells are different in structure and func- 
tion. A further differentiation leads to specialization, 
and a division of labor among the cell complexes or 
tissues occurs. Each tissue is simple but specialized. 
When specialized tissues grow into each other, compound 
tissues arise. 

Ontogenetically the specialization of tissues (histo- 
genesis) is direct, but the development of the differentia- 
tions is gradual, representing a repetition of phyloge- 
netic process. The relation of the germ layers to 'the tissues 
has not yet been finally established, but it may be per- 
missable to give the following survey of the histogenetic 
function of the germ layers of the Vertebrates: 

Ectoderm : External epithelium and epithelium of the 
cuticular glands, nerve tissue, sensory cells. 

Entoderm : Epithelium of the intestine, of its append- 
ages and glands, chorda, epithelium of blood vessels and 
blood (?) 

Mesoderm : Epithelium of the body cavity, germ epi- 
thelium, glandular epithelium of the kidneys, muscular 
tissue, connective substance. 

Modes of Reproducton. Reproduction is the goal 
of all active existence. The lower an organism is the 
more pregnant becomes this fact. Posterity is procured 
in many different wa}^s. We mention sexual reproduc- 
tion first. An animal may either produce large masses 
of very small eggs so that many small and imperfect in- 
dividuals are born which have to undergo a number of 
changes before they reach maturity (metamorphosis), 
or it may produce few, but large eggs, rich in yolk, 



MODES OF REPRODUCTION. 67 

which bring forth individuals comparatively well devel- 
oped {without metamorphosis) . The existence of a new 
individual is still more fully assured wherever special 
care in the parental body or under parental supervision 
exists. The chief mode of reproduction among Metazoa 
is sexual reproduction. It requires a mixing of individ- 
uals and thus becomes necessary for every species. We 
distinguish animals with separate sexes from those in 
which both sexes are in one individual, called hermaphro- 
dites. The latter phenomenon is a secondary adaption 
to new conditions. In both cases the crossing of two in- 
dividuals is the normal requirement. Many lower ani- 
mals simply eject their sexual products into the water, 
leaving the union of male and female form to mere 
chance. The higher animals pass through a compli- 
cated process of copulation, which saves reproductive 
material. Between these two modes there are a great 
many modifications. Sometimes the spermatozoa alone 
are ejected into the water which carries them into the 
maternal body (into the tissues of the Sponges, into the 
ovaria of Chrysaora, into the gills of some Mollusca). In 
the transition towards copulation male and female live 
together. Whilst the latter deposits the eggs the male 
covers them with spermatozoa (Teleostei). External 
copulation takes place among the frogs. Genuine or in- 
ternal copulation is usual among all terrestrial and many 
aquatic animals. Most insects keep the spermatozoa in 
the receptaculum seminis until the egg is ripe for ferti- 
lization and ready to be discharged. Reptiles and birds 
lay their eggs during the first stages of segmentation. 
Those of the birds cease to develop until incubation be- 
gins. 

The organs of copulation consist in the male of a 



68 MKTAZOA. 

vesicula seminalis, a ductus ejaculatorius and (mostly) 
of a penis : in the female of a vagina (frequently together 
with a receptaculum seminis). The modifications of the 
penis of some Crustacea, the spiders and the Cephalo- 
poda, deserve special mention. The distinctive charac- 
teristic (dimorphism) of male and female are largely 
confined to the reproductive organs, but other modifica- 
tions (of size and color) arise when the conditions re- 
quire such. The most interesting investigations of this 
kind have been carried on by Kowalevsky and Darwin 
(dimorphism of Bonellia and heterogeny of certain Nema- 
todes) . 

Parthogenesis or reproduction from unfertilized eggs is 
derived from sexual reproduction without fertilization. 
According to Weissmann, only one polar body develops, 
instead of two. Occasional parthenogenesis has been 
observed among the butterflies. Normal parthenogenesis 
takes place, e. g., in the honey bee, whose drones are 
developed from unfertilized eggs. In Cladocera and 
Ostracoda an alternation of parthogenetic and fertilized 
eggs occurs. Heteroparthenogenesis is found among the 
Aphides, where two distinct female individuals exist. 
Those with wings are parthenogenetic ; those without 
wings reproduce sexually. Paedo-parthenogenesis occurs 
wherever parthenogenesis and paedogenesis (larval sexual 
maturity) concur as observed in the larvae of Cecidomyia; 
also certain developmental phenomena of Distomea may 
be explained on the same basis. The cause for parthe- 
nogenesis is largely to be explained on economic prin- 
ciples. 

Reproduction by fission frequently among the lower 
Metazoa. The organism divides into two parts of sim- 
ilar size, in each of which the necessary organs are 



MODKS OF REPRODUCTION. 69 

developed by regeneration. The power of regeneration 
is the ability to replace lost members of the body. The 
lower the differentiation of an organism the stronger the 
tendency for regeneration. Trembley's investigations of 
last century show the extraordinary power of regen- 
eration of the fresh-water Hydra. We may cut a Tur- 
bellarian or Annulate worm into two parts, and each 
one will grow into a complete animal again. Similar 
regenerations occur among the Crustacea and Echino- 
dermata; even the Vertebrates exhibit such phenomena, 
e. g. , in the growth of the severed extremities of Triton 
and lizard, and in the replacement of finger-nails and 
the healing of wounds. 

The cells along the cut surface give up a part of their 
differentiation, grow very rapidly and furnish material 
for new limbs, but always rebuild the old germ layer to 
which they originally belonged. The division may be 
artificial; we tear a starfish into two parts and regenera- 
tion follows. In the genus Nais regeneration precedes di- 
vision, even to the partial formation of new organs. Re- 
generation zones arise whenever a new individual begins 
to appear; division takes place as soon the new forms are 
able to perform the necessary functions, each one de- 
veloping a new zone before separation, so that sometimes 
temporary cormi arise. This constitutes an incomplete 
division resembling budding. The process of division 
only occurs where there is no sexual maturity. Embry- 
onic division, with premature regeneration, takes place 
when the adult animal is too complicated to permit di- 
vision. We find this phenomenon even in man in the 
development of twins, which may result either from the 
fertilization of two eggs or from the division of an early 
embryonic stage. Incomplete division causes monstrosi- 
ties. 



70 MKTAZOA. 

A number of other phenomena, not yet fully investi- 
gated, belong under the head of fission. 

Reproduction by budding occurs when only a limited 
part of the adult body develops into a new individual. 
Primordial budding takes place especially among" the 
Cnidaria and the Sponges. An evagination ol the body 
wall arises (anywhere), grows in length and assumes the 
form of a young Hydra, with tentacles and mouth open- 
ing. It sometimes separates from the mother and be- 
comes an independent individual; when it remains cormi 
arise, which may branch out like trees. Often the bud- 
ding individuals differ from the sexual individuals, being 
merely persisting larvae. Continued embryonic budding 
occurs in more highly complicated organisms and only 
in certain predisposed places. It begins in the embryo 
when a primary bud arises, from which all later buddings 
are developed. Otherwise it proceeds like primordial 
budding. This phenomena has been observed among 
the Bndoprocta, Bctoprocta and many Tunicata. Both 
regeneration and budding may be due to similar causes. 

It has been stated before that the various modes of 
asexual reproduction occur only as secondary modifica- 
tions together with sexual reproduction (often in the 
larval stages of an animal), in such a way that the for- 
mer regularly alternate with the latter. An alternation 
of geyieration takes place whenever two generations with en- 
tirely different modes of reproduction and of entirely dif- 
ferent organization follow each other. Even the phenomena 
of heterogeny might be classified under this definition. 

Alternation of generation is therefore based upon : 

(a) Heterogeny (e. g., Rhabdonema nigrovenosa). 

(b) Parthenogenesis (e. g., Aphides, Distomea). 

(c) Fission*(e. g., Scyphomedusae). 



THEORY OE HEREDITY. 7 1 

(d) Budding (e. g., Hydroniedusae). 

The most peculiar combinations arise during these 
changes. The individuals of a cormus may perform the 
same functions (homomorphous), or each one may per- 
form a distinct function (polymorphous). The cormus 
of the Siphonophora consists' (according to Leuckart) of 
locomotory individuals, nourishing individuals, repro- 
ducing individuals, etc., thus resembling only organs of 
one and the same individual. 

The considerations of the phenomena of reproduction 
may justify a few remarks on the theory of heredity, by 
which the causal relation is to be explained which 
exists between the morphological phenomena of one 
generation and those of a following one. Its aim is to 
interpret not only the repetition of typical character- 
istics, but especially the appearance of new ones, and 
the various ways in which they are transmitted to suc- 
cessive generations. The first problem has been ex- 
plained by the theories of differentiation (which deals with 
the changes of the reproductive cells) and of fertilization 
(which deals with the mixing of characters by the union 
of two individuals). 

The theory of differentiation explains both the develop- 
ment of the organism from the egg, and the reappearance 
of reproductive cells in the new organism. Caspar 
Friedrich Wolff 'first elucidated the process of differentia- 
tion in the egg of the chick (theory of epigenesis, 1759). 
In modern times three prominent scientists claim to have 
found the true interpretation of the problem : Darwin, 
in his pangenesis, Ndgeli in his theory of idioplasm, 
and Weissma?i?i in his theory of the co?itinuity of the 
germ plasm. 

Darwin assumes that minute germ particles of all the 



72 - M^TAZOA. 

cells, constituting the various tissues are carried to trie 
generative organs and deposited in the reproductive 
cells, so that all the qualities of the body exist there, as 
we might say, side by side. During development they 
are again distributed to the corresponding parts of the 
new body. It is difficult to show how that process is 
carried out. 

Nageli assumes the existence of a more or less compli- 
cated plasma structure, the idioplasm, which extends as 
a connected network through all the cells of the body. 
It consists of a very large number of elementary parti- 
cles, micella, which posses all the qualities of the indi- 
vidual. The manifold differentiations of the various 
parts of the body are caused by the action of certain de- 
finite rows of similar micellae whilst the others are at 
rest, so that potentially all the cells of the body possess 
the same qualities, but only a few of them manifest them- 
selves at a time. Thus all the qualities of the body are 
present also in the egg cell. 

Weissmann maintains that it is a definite part of the 
cell which causes differentiation, namely the nucleus. 
He applies the name idioplasm to the nuclear substance 
(chromatin), but defines it in a different way. Nuclei 
are separate formations with separate distinct qualities. 
In the nucleus of the reproductive cell, however, are all 
the qualities of the various nuclei present, side by side. 
During development these qualities are distributed to 
the various cells of the new body. The karyoplasm of 
the reproductive cell (W .' s germ plasm) possesses, there- 
fore, the most complicated structure, whilst that of the 
other cells represents the simplest structure. Some of 
the cells receive the original nuclear substance and are 
deposited as reproductive cells in the new body (conti- 



Theory oe heredity. 73 

nuity of germ plasm). The different kinds of karyo- 
plasmatic formations are consequently autogenetically 
the same, resembling Nageli's idioplasm. 

Hatschek holds, chiefly on the basis of the phenomena 
of regeneration, that not a distribution, but an actual 
change of qualities occurs which is due to the infinite 
possibilities of variation characteristic of the chemism 
of organic combinations. He explains the origin of the 
reproductive cells from the fact that in every organism 
virtually undifferentiated cells are deposited to furnish 
the reproductive cells (continuity of virtual germ cells). 

The theory of fertilization has established the fact that 
conjugation is not a mere influence of the spermatozoon 
over the ovum, but an actual continued existence of its 
organization in the fertilized ovum and its products, 
which implies the transmission of the characteristics of 
both parents, each furnishing one half. This diminishes 
in successive generations. Phenomena of atavism, the 
barrenness of bastards, grafting of hybrids, etc., are of 
importance. 

The transmission of individual characteristics may be 
(according to Darwin) either direct, affecting a part or the 
whole of an organism immediately, or indirect, affecting 
the reproductive cells only and manifesting themselves 
in the following generation. Darwin explains the pro- 
cess according to his theory of pangenesis and Nageli 
according to that of idioplasm. Weissmann, on the other 
hand, maintains that acquired or direct changes of the 
body are not transmissable, only those which affect the 
germ cells. He explains this on the basis of his theory 
of the continuity of the germ plasm. A transmission of 
injuries has never been proved; diseases are transmitted, 
from generation to generation, by infection. Special 
E 



74 M3TAZ0A. 

talents manifest in many generations may be traced back 
to the first ancestor of the family. Weissmann also shows 
that the assumption of direct transmission is not neces- 
sary for the explanation of phylogenetic changes. As a 
consequence of the continuity of the germ plasm it is 
but natural to suppose that all phenomena which effect 
the body effect in some indefinite way the reproductive 
cells, producing in them latent changes, which manifest 
themselves in the following generation. A change in the 
constitution of the reproductive cell conditions a change in 
the constitution of every body cell, i. e., of the entire body. 



CHIEF CLASSIFICATION OF THE 
METAZOA. 



II. Zoophyta or Ccelenterata. 

III. Plathelminthes. 

IV. Vermes. 

V. Arihropoda. 
VI. Mollusca. 
VII. Echinodetmata. 
VIII. Tunicata. 
IX. Vertebrata. 



ZOOPHYTA OR CCELENTERATA. 



The body consists of two layers, ectoderm and ento- 
derm. An intermediate layer is either absent, or, when- 
ever it is present, it shows intimate relations to ectoderm 
and entoderm. The digestive track has only one ex- 
ternal opening (mouth). A body cavity between intes- 
tine and cuticle, as well as blood vessels and excretory 
organs, are absent. A nervous system is either entirely 
absent or is but little centralized. 

I Class. Gas traeada* (showing essentially the struc- 
ture of a gastrula). A. Physemaria. Tubular beings, 
consisting of two layers with one aperture, fastened to 
the bottom of the sea. Generative organs develop in 
entoderm ; the entoderm contains foreign bodies. Hali- 
physema, Gastrophysema. A. Dicyemida (parasites in 
Cephalopoda, Kchinodermata and Turbellaria). Ecto- 
derm ciliated, forming a continuous layer around the 
solid entoderm, which consists of a single multi-nuclear 
axillary cell, no mouth opening, no digestive tract. 
Generative organs consisting only of ova-like germs 
which develop within the axillary cell apparently with- 
out fertilization. The unicellular germ divides into two 
unequal parts. The larger one (macromere) remains 
undivided and becomes the axillary cell; the smaller one 

*K- van Beneden and Julin suggest the name Mesazoa for this 
class, making it an intermediate series between Protozoa and 
Metazoa; whilst I/uckart, MetschuikofF and Whitman consider 
them to be retrograded Plathelminthes. 



PORIFKRA. 77 

(micromere) repeatedly divides and becomes the ecto- 
derm. Dicyema. C. Orthonectidcs . Entoderm consists of 
a layer of cells; body externally segmented. A layer 
of muscular fibres between ectoderm and entoderm. 
Sexes separate. Spermatozoa and ova in entoderm. 
The micromeres develop the muscular fibres. The mac- 
romere forms the accumulation of cells. Rhopalura. 
Appendix. Trichoplax adherens, peculiar animal dis- 
covered in the marine aquarium at Gratz, consisting of 
three layers, and resembling a ciliated plate of irregular 
shape. It multiplies by fission. 

II Class. Porifera or Sponges. 

i. Subclass: Calcaria. Skeleton always present con- 
sisting of calcareouss picules. Asconidce (with simple 
canals): Olynthus. Grantia. Syconidtz (thick walls 
with straight radial tubes): Sycandra. Leuconidcz (thick 
walls with branched channels): Leuca?idra. 

i. Order: Calcispongice . 

2. Subclass: Non-Calcarea. Silicious skeleton. In 
structure they are L,euconid3e. 

2. Order: Hexactinellidce. (Glass sponges). Silex 
spicules either isolated or forming a hyaline lattice- work 
of six-rayed stars, resembling the radial tubes of the Sy- 
conidae. Cemented together by a silicious substance. 
Mostly fossil. Living: Euplectella (Philippines); Hya- 
lonema (Japan). 

3. Order: Spiculispongicz (needle sponges). Skeleton 
consisting of independent, different silicious spicules ; 
rarely absent. Cemented together in bundles by an or- 
ganic substance or forming a massive, firm network. 
Never cemented together by a silicious substance. Geo- 
dia, Ckondrosza, Oscarella and Halisarca (without skele- 
ton). Tethya, Tuberella, Suberites. 



78 ZOOPHYTA OR CCKly^NTKRATA. 

4. Order: Halichondrince : Skeleton consisting of 
uniaxal silicious needles cemented together by a horny 
substance (spongine). Halichondria, Reniera } Spongilla 
(fresh water), Myxilla, Clathria. 

5. Order: Ceraspongice (horny sponges). Skeleton 
consisting of horny fibres. No spicula of their own. 
Often grains of silex and sand are present as foreign 
bodies. Spongelia, Euspongia officinalis (common toilette 
sponge), Aplysina. 

All sponges with the exception of the Spongillidae 
live in the sea. Their external form and structure is so 
variable that a general description is impossible. Their 
internal structure is represented by three types, the 
Asconidcz, Syconidce and Leuconida. The first type (e. g., 
Olynthus) consists of a thin- walled tube, attached at one 
end and open at the other. Its walls is perforated by 
pores, which may open or close whilst the water flows 
through them in the tube and out through the aperture 
or osculum. It consists of two layers: of an almost homo- 
geneous external layer in which cells and calcareous 
needles are imbedded and an internal epithelium of col- 
lared cells resembling the protoplasmic collar of certain 
Flagellata. There may be a thin flat epithelium exter- 
nal to the layer containing the skeleton so that we would 
get three layers, an ectodermal, an entodermal and a 
mesodermal layer of connective tissue. In the second 
type (e. g., Sycandra) the walls have become thicker and 
radial tubes penetrate them from the central cavity so 
that the surface of the sponge shows numerous cone 
shaped elevations above these tubes. They are lined 
with a collar-epithelium, whilst the epithelium of the cen- 
tral or gastrula cavity seems to be changed into a pave- 
ment epithelium. The water penetrates tlie radial tubes. 



PORIFBRA. 79 

enters the gastric cavity and makes its exit through the 
osculum. The third type (L,eucandra), representing the 
large majority of sponges, shows a still more complicated 
canal system. The collar-epithelium is confined to the 
so called flagellate chambers lying in the thick mesoderm. 
The pores open into branching canals covered with flat 
epithelium, which lead into the flagellate chambers (ad- 
ductory canals). They in turn open into another set of 
branching canals (eductory canals), which lead into the 
central cavity and finally to the osculum. 

The vibration of the flagella of the collar-epithelium 
keeps up a constant flow of water in the canal system of 
the sponges. A highly- developed canal-system indicates 
a loose structure of the body, a highly, developed meso- 
derm always indicates a solid structure of the body. 

The mesoderm of the sponges represents a kind of con- 
nective tissue consisting mostly of a gelatinous consist- 
ence in which variously shaped cells are imbedded. These 
often contain pigment or possess the power of motion 
(wandering cells), or contraction (closing the pores, 
muscular cells). The mesodermal tissue is the seat of 
the various skeleton formations (silicious, calcareous, 
horny). There may be one or more kinds of spicula in 
one individual, occurring either loose or connected into 
skeletons. The same is true of horny fibres. The com- 
mon toilette sponge is simply the horny skeleton of the 
animal with the animal parts removed. 

A nervous system has not yet been discovered (meso- 
dermal ganglion cells?). 

Reproduction is either sexual or asexual. 

Asexual reproduction takes place by external or inter- 
nal gemmation or budding. 

External budding. Buds are formed at different parts 



80 ZOOPHYTA OR CCEXENY 3R ATA . 

of the body, which, by budding, may give rise to stocks 
or colonies. Pseudo-canals arise within the interspaces, 
which must be distinguished from the true canals. The 
number of oscula may correspond to the number of 
indi yiduals constituting a stock. 

Internal budding (? ) Groups of cells {gemmulce) sepa- 
rate from the body and develop, after an interval of rest, 
into sponges. (Spongilla.) 

Sexual reproduction. The sponges are largely herma- 
phrodites or dioecious; however, spermatozoa and ova 
are developed at different times ; they are proteranderous 
hermaphrodites. These cells seem to be developed from 
mesodermal cells. 

Development. Oscarella (Halisarcd) lobularis mani- 
fests the following typical phenomena. After repeated 
divisions of the ovum a free swimming larva or blastula 
is produced, a spherical form consisting of a single layer 
of flagellate cells. Through invagination a gastrula 
arises which attaches itself at the blastopore, or mouth- 
end, which gradually closes. Between ectoderm and en- 
toderm a gelatinous substance is secreted into which cells 
migrate, probably from the ectoderm. Thus the meso- 
derm (connective tissue) arises. Radial evaginations of 
the entoderm develop from the ccelom, and grow into the 
mesoderm, becoming flagellate chambers. They connect 
with the surface, either by pores or by evaginations of the 
ectoderm. The osculum is formed at the aboral pole by 
the lengthening of the ccelom, which finally breaks 
through : Sycon stage. 

The relation between the Porifera and the other Ccelen- 
terata is only indirect, since the osculum does not corre- 
spond to the blastoporus of the gastrula, nor to the mouth 
of the Ccelenterata. 



• CNIDARIA. 8 1 

III Class. Cnidaria. 

i . Subclass : Hydrozoa. Prototype Hydropolyp or Hy- 
drate. Mouth leads directly into the entodermal canal. 
No gastric filaments. Genital products arise from the 
ectoderm. Sexes represented by different persons. 

i. Order: Hydridtz (fresh water polyps) . Single per- 
sons or small cormi without periderm, consisting of a 
few persons of the same kind. Reproduction, sexual 
and asexual (budding). Hydrae develop directly from 
the egg. Hermaphrodites. Hydra, Microhydra in fresh 
water. 

2. Order: Hydromedasce. Hydroid cormi, at least 
dimorphous, since beside the usual sterile nutritive per- 
sons sexual persons arise by budding, which separate 
either as craspedote free swimming medusae or remain 
sessile as medusoid gonophores. In a number of Hydro- 
medusae the attached Hydroid form is suppressed, inas- 
much as from the fertilized eggs of the craspedote medu- 
sae, new medusae are directly developed. . A natural 
classification of this order is an impossibility since the 
observations are still incomplete. 

i. Suborder: Hydrocorallicz. ^Colonies with calcareous 
peridermal skeleton. Genital products develop in the 
gonophores. Stylaster. Millepora. No corresponding 
medusae. 

2. Suborder: Tubateria. Naked or chitinous colo- 
nies. The chitinous periderm never widens into a cup- 
shaped cell around the polyp head. In some forms the 
medusae are reduced to sessile gonophores. The corre- 
sponding A7ithomedus<z are craspedote medusae. With- 
out marginal bulbous swellings and without otoliths; 
ocelli at the base of the tentacles. Gonads in the external 
wall of the digestive stalk ; 4, rarely 6 or 8, radial canals. 



82 



ZOOPHYTA OR COEXKNTKRA'M.. 



The following examples represent the Hydroid form on 
the one side and the corresponding medusae form on the 
other: 

Syncoryne Sarsii. Sarsia tubulosa. 

Podocoryne carnea. Dysmorphosa camea. 

Eudendrion ramosum. Lizusa octocilia. 

Bougainoillea ramosa. Margelis ramosa. 

Stauridhim Cladonema. Cladonema radiatum. 

Codylophora lacustris (fr. w.) Wanting. 

Tubularia larynx. Wanting. 

Unknown. Ctenaria ctenophora. 

3. Suborder: Campanaria. Hydroid colonies with 
chitinous periderm, enlarging around the polyp heads 
into Campanula into which the heads with the tentacles 
are withdrawn. Modified polyps, without tentacles and 
mouth (Qpnangia) develop medusa buds or sessile gonc- 
phores, united in groups. The corresponding Lepto- 
medusez are craspedote medusae, partly with and partly 
without bulbous swellings along the edge, when present; 
they are developed from the velum insertion, with exo- 
dermal otolith cells. Ocelli, either absent or present, 
along the base of the tentacles. Gonads always along 
the radial canals. Number of radial canals variable. 

Campanularia geniculata. 

Unknown. 

Unknown. 

Campanidina termis. 

Unknown. 

Laomedea caliculata. 

Canaliculata. 

Related to the Campanaria are the Plumularice and the 
Sertulariee. They are branched Hydroid colonies. The 
nutritive polyps of the former are arrangedjn single rows; 
those of the latter in double rows, on opposite sides of the 
stem. The genital products are budlike processes (gono- 



Obelia geniculata. 
Eucope canipanulata. 
Gastroblasta Raffcelii. 
Phialidium variabile. 
sEquorea Forskalea. 
Wanting. 



CNIDARIA. 83 

phores) (with a chitinous periderm) which arise in groups, 
on specially modified polyps, without mouth and tent- 
acles. Their nature is not fully understood. 

4. Suborder: The Hydroid form is wanting. The cor- 
responding TrachomeduscB are craspedote medusae with 
auditory vesicles which may either project freely on the 
marginal surface above the velum or be enclosed in a 
vesicle which lies in the gelatinous substance of the disc 
and close to the edge of the latter. Ocelli mostly want- 
ing. Gonads always along the radial canals. Either 4 
or 6 or 8 radial canals, between them often blind centri- 
petal canals. Direct development with metamorphosis. 
Olindias Miilleri. Rhopalo7iema velatum. Aglantha digi- 
talis. Geryonia proboscidalis. Carmarina hastata. 

5. Suborder: Hydroid form wanting. The corres- 
ponding Narcomeduscz are craspedote medusae with audi- 
tory vesicles which always project freely on the margi- 
nal surface and contain entodermal otolith cells. Ocelli 
mostly wanting. The tentacles are inserted upon the 
exumbrella which is thus divided into a number of col- 
lar flaps. Gonads in the digestive stalk extending pe- 
ripherally into the radial digestive pouches. Radial 
canals either wanting or present in the form of flat radial 
digestive pouches. Circular canal sometimes obliterated. 
Number of tentacles, flaps and pouches indefinite (4-32). 
Development direct with metamophosis. Cunina, Peg- 
antha. ALgineta. sEginopsis. Solmaris. 

3. Order : Siphonophorcs : Polymorphous, free swim- 
ming Hydrozoan colonies, whose persons are modified 
craspedote medusae adapted to special functions. 

1. Suborder: Siphonanthce . The heteromorphous per- 
sons bud along a stalk of variable form, resembling the 
digestive stalk of a medusa. 



84 ZOOPHYTA OR CXELENTERATA. 

i. Family: Calyconectce without pneumatophore and 
tentacle with one or more swimming bells at the upper 
end of the stalk. The remaining heteromorphous per- 
sons arranged into groups (cormidia), which may detach 
themselves either as Eudoxia or Ersaea. Praya, Diphyes. 
A by la . Hippopodius . 

2. Family: Physonedce with pneumatophore, without 
aurophore, with several swimming bells and tentacles. 
Apolemia. Agalma. Anthemodes. Halistemma. Physo- 
phora. Forskalia. 

3. Family: Auronectcz with a large pneumatophore; 
beneath it a circle of swimming bells in whose dorsal 
median line a large (respiratory) air bell (aurophore) 
lies (a modified swimming bell). Stalk shortened and 
thickened. Without tentacle(?). Stephalia. Aurelia. 
R hod all a. 

4. Family: Cystonecta with a large pneumatophore 
without aurophore. Swimming bells and hydrophyllia 
(protections) wanting. Rhizophysa Physalia (very short 
stalk, disc-like, thickened). 

2. Suborder: Disconanthce. The heteromorphous per- 
sons are situated on the subumbrella of a disc which en- 
closes a many chambered pneumatophore and resembles 
a medusoid umbrella. The margin of the disk carries a 
circle of numerous tentacles. In the middle of the sub- 
umbrella is the central nutritive stalk or chief sipho. 

5. Family : Disconectce. Discalia. Porpita. Porpalia. 
Velella. 

II Subclass : Scyphozoa. Prototype: the Scyphopolyp 
or the Scyphula. Always with ectodermal oesophagus. 
Gastric or mesenteric filaments are present everywhere 
along the septa. The genital products arise from the 
entoderm. The sexes are generally represented by dif- 
ferent persons. 



CNIDARIA. 85 

i. Order: A?ithozoa (corals). Separate sessile persons 
or colonies. Body essentially a Scyphnla. The ecto- 
dermal oesophagus leads in form of a tube into the wide 
gastric cavity which is divided into a number of periph- 
eral diverticula (radial pouches) separated by septa. 
The septa continue with their free internal margin to- 
wards the aboral part of the body. 

I. Suborder .A. Octocorallia. Alcyonaria (or Actino- 
zoa). Generally with 8 septa and 8 tufted tentacles. 
Polyp colonies of very varying form. Skeleton forma- 
tion very different. Alcyonium, Permatzda. Kophobe- 
lemnon. Goigonia. his. Tubipora 

B. Tetracorallia. .2. Suborder : Rugosa. Number of 
septa large, a multiple of 4. With calcareous skeletons. 
Fossil paleozoic forms. 

C. Hexacorallia. 3. Suborder: Antipatharia (horny 
corals). With 6 or 24 simple tentacles. Colonies with 
a horny auxiliary skeleton. Antipathes (6 tentacles, 2 
developed septa), Gerardia (24 tentacles and septa.) 

4. Suborder: Madreporaria (stony corals). Mostly 
colonies, rarely single, with strongly developed calcareous 
skeleton. 6n simple tentacles and septa in larger num- 
ber and variable arrangement. Madrepora. Astroides 
Fungia. Astrcea. Mceandrina. Cladocora. Caryophyl- 
lia. Flabellum. 

5. Suborder: Actinaria (fleshy corals.) Mostly single, 
with 611 tentacles and septa in large numbers, and dif- 
ferent arrangement. Without skeleton. Cerianthus. 
Zoa?ithus. Actinia. Anemonia. Adamsia. Edwardsia. 

II. Order: Scyphomedustz (Acraspedae). Mostly free 
swimming single persons, bell or disc-shaped ; the mes- 
odermal supporting layer developed into an enormous 
gelatinous mass. The ectodermal oesophagus lies mostly 



86 ZOOPHYTA OR COUNTER ATA. 

within the digestive stalk, which is suspended from the 
centre of the subumbrella. The four radial pouches of 
the Scyphula are retrograded in the higher forms. The 
exumbral and subumbral walls of the gastric cavity fuse 
in such a way as to form only a system of radial gastro- 
canals of different structure. With tufts of gastric fila- 
ments Genuine velum wanting ; instead marginal lobes 
with prolongation of the gastro- vascular system. 

A. Medusae with highly arched umbrella; the 4 
radial digestive pouches and their separating septs more 
or less distinctly preserved. 

1. Suborder: Stauromedusa 4 septa preserved (JLu- 
cernarid) or reduced to 4 knots {Tessera), 4 or 8 gonads 
in the subumbral wall of the 4 digestive pouches ; with- 
out sensory vesicles. Lucernaria (sessile 8 marginal 
lobes, each carrying a tuft of tentacles). Tessera (free 
without distinct marginal lobes, with 8 tentacles). 

2. Suborder: Peromeduscs. 4 septa reduced to 4 knots, 
therefore the 4 pouches united into a circular sinus. 8 
gonads at the subumbral wall of this sinus. With 4 in- 
terradial sensory vesicles. 8 or 16 marginal lobes, 4 or 
12 tentacles. Pericolpa. Perlphylla. 

3. Suborder: Cubomedustz (Chary bdeidcs) . 4 septa pre- 
served; 4 pairs of gonads on the septa, freely protruding 
into the digestive pouches. With 4 perradial sensory 
vesicles, containing an auditory vesicle with an ento- 
dermal otolith sac and carrying one or more eyes ; 4 in- 
terradial tentacles or tentacle tufts. Mostly with vela- 
rium. Charybdea, Chirodropus. 

B. Medusae with flat discoid umbrella. .The four 
primary digestive pouches of the Scyphula retrograded 
through disappearance of the septa. Instead there are 
secondarily developed (as a remnant of the fusion of the 



CTEXOPHORA. 87 

exuinbral and subumbral vascular-lamella) 8, 16, 32, or 
more broader or smaller, often anastomosing radial canals. 
The 4 interradial septa (remnant of the original septa) 
or taeniola carry the phacella or tufts of gastric filaments. 
Development either directly with metamorphosis or with 
alternation of generations. In the latter case the gas- 
trula develops into a sessile Scyphula which becomes a 
young sessile medusa (Scyphistoma). The Seyphis- 
toma is capable of reproduction by fission or budding 
(strobilation). The constricted medusae (Ephyra) change 
by metamorphosis into the adult form 

4. Suborder: Discomednsce. 1. Family: Cannostomce. 
With simple mouth tube, without oral arms, with square 
mouth and short solid tentacles. Nausitho'e. 2. Family: 
SemosiomcE. With four large multi-lobed oral arms and 
cross-shaped mouth. With long, hollow tentacles. 
Pelagia noctiluca. Cyanea. Aurelia aurita. 3. Family: 
Rhizostoma. Mouth closed. With numerous suctorial 
mouths on the 8 long root like oral arms, without ten- 
tacles. Cassiopea. Pilema {Rhizo stoma). Cotylorhiza. 
Crambessa. Ca?ino?'hiza. 

IV Class. Ctenophora. 

1. Order: Tentaculata. With two lateral tentacles 
which can be retracted into pouches. Gastro-vascular 
canals end blindly. 1. Famify : CydippidcB. Body 
spherical or oval. Hormiphoi'a. 2. Family: Lobatcz. 
Body laterally compressed with two oral lobes in the 
median plane. 

Eucharis. 3. Family: Cestidce. Body elongated to the 
form of a band in the direction of the sagittal plane; 
without oral lobes. Cestus. 

2. Order : Nuda. Without tentacles. Mouth wide, 
oesophages very large ; gastro-vascular canals strongly 
anastomosed. 4. Family : Beroidcz. Bero'e. 



88 ZOOPHYTA OR CCELKNTKRATA. 

General Description. The Cnidaria are Metazoa with a 
persisting primary axis, and of radial structure ; with 
epithelial muscle and nerve tissue, and with nemato- 
cysts (nettling cells or lasso-cells); their germ epithelia 
arise either from the extoderm or entoderm. Their 
ancestral type is polypoid (Archhydra of Haeckel). 

We distinguish two fundamental forms of structure : 
the sessile polyp form and the free swimming medusa 
form. Some Cnidaria become sexually mature, as 
polyps, others later in their medusa condition ; the 
transition takes place by means of an alternation of gen- 
eration. Phylogenetically the sessile form is the earlier 
from which the medusa form has been derived by adapt- 
ation to the free swimming mode of life and by higher 
differentiation. 

The body of the polyp has the form of a tube, grown 
fast to its support at its basal end {pole of attachment), 
whilst the other free end shows the mouth opening (oral 
pole). Near this end is a circle 0/" mobile, very contractile 
tentacles, which thus divides the body into two regions, 
the oi r al disc and the aboral disc or the cup. 

Two anatomically distinct types are known. 1. The 
Hydropolyp or Hydrula. A primary mouth opening leads 
directly into the very simple primitive gastric cavity 
which continues only within the tentacles. The body 
Wall consists of the ectodermal epithelium and the ento- 
dermal epithelium, which join at the mouth opening : 
between them lies a gelatinous layer, free from cells. 
The same layers are in the tentacles. 2. Scyphopolyp or 
Actinopolyp. A secondary mouth opening leads into an 
oesophagus, which is internally covered by the ectoderm 
(evagination of body wall); at the internal oesophageal 
opening, ectoderm and entoderm join. The primitive 



CTKNOPHORA. 89 

gastric cavity (the gastro- vascular cavity) is more com- 
plicated on account of its projecting longitudinal ento- 
dermal folds {septa), so that we distinguish in it a central 
stomach and peripheral gastric canals. In the oesophageal 
tube the septa are broader and mostly grown together 
with it, so that the gastric canals continue here as gastric 
pouches, reaching into the tentacular cavities. The septa 
are situated between every two tentacles. The body 
wall consists of three layers, the gelatinous layer, how- 
ever, containing mesenchymal cells (connective tissue 
cells). 

The body form of a medusa corresponds in a general 
way to that of an arched disc or bell. The convex aboral 
surface (cup of polyps) is uppermost, it has the name ex- 
umbrella. Around the margin of the disc is a circle of 
tentacles (homologous to that of polyps). The lower con- 
cave surface, called subumbrella, is homologous to the 
mouth disc of the polyp; it carries in its centre a stalk- 
like process {digestive stalk, mouth stalk) at whose lower 
end the mouth is situated. The gelatinous layer has 
thickened into a powerful disc {iimbrella) lying beneath 
the exumbrella. The gastro-vascular system is thus 
pressed closely to the surface of the subumbrella. In 
this connection it is generally so transformed that only 
its central part remains a cavity {central stomach), whilst 
its peripheral part forms a system of canals, mostly radial 
canals and a marginal ring canal into which the tentacu- 
lar canals open. 

The exumbrella is covered with a simple pavement 
epithelium. The margin of the disc does not only bear 
the tentacles, but also the epithelial nervous system 
(double ring of ganglionic nerves in Hydromedusse, a 
number of ganglia in Scyphomedusae) and the numerous 



90 ZOOPHYTA OR CCElvENTERATA. 

sensory organs (auditory organs, eyes), It also has in 
the Hydromedusae a contractile marginal border or velum 
beneath the tentacles which narrows (like a diaphragm) 
the entrance to the bell cavity; in the Scyphomedusae, 
however, it is prolonged into a number of marginal lobes. 
The subumbrella possesses a strong muscular epithelium 
by whose (and that of velum) contraction the water is 
rhythmically ejected from the bell cavity. The reac- 
tion propels the animal in the direction of the aboral 
pole; the elastic gelatinous disc acts as an antagonist to 
these muscles. 

Two types of medusae are, therefore to be distinguished 
the Hydromedusae and the Scyphomedusae. However 
much similarity there is in the structure, a closer mor- 
phological investigation shows that they do not descend 
from one and the same type, but the one from the Hy- 
dropolyps the other from the Scyphopolyps. The Cni- 
daria are therefore divided into two chief divisions, the 
Hydrozoa and the Scyphozoa each of which contains 
earlier polyp forms from which medusae forms originated, 
The germ epithelia of the Hydrozoa are ectodermal, those of 
the Scyphozoa entodermal. 

The histology of the Cnidaria has been indicated under 
the general head of the Metazoan histology. 

The characteristic radial structure of the Cnidaria is 
based upon a repetition of like organs around the chief 
axis. The intercalation of new radii, which may be 
either regular or irregular, is the law of growth govern- 
ing the multiplication of the radii. 

Reproduction and development. The sexes of the Cnida- 
ria are, with a few exceptions, separate. The genital 
products are in most cases simply emptied into the 
water, The eggs are generally very small and very 



CTENOPHORA. 91 

numerous. Segmentation is equal. Gastrulation occurs 
either by polar invasion or by invagination or by epibol- 
ism. The typical larva of the Cnidaria is the so-called 
planula, a long, oval, ciliated form consisting of an exter- 
dal ectoderm epithelium, and an internal compact ento- 
dermal mass, without protostoma and coelom. In this 
condition the larvae does not take any nourishment, and 
is capable of rapid locomotion. The planula fastens 
itself with the apical pole, and changes into a polyp, 
which represents either the final form (Actiuozoa) or 
produces by fission or budding medusae (Hydromedusae, 
Scyphomedusae). In some cases medusae arise directly 
from the planula (Haplomorpha, Pelagia). Reproduc- 
tion by {primordial) budding or fission, is very general, 
and may take place in the planula stage, but most 
frequently in the polypoid stage. A bud arises by 
an evagination of all the layers of the body wall, form- 
ing either polyps or medusae ; medusae, however, only 
produce medusae by budding. The new individuals 
may either detach themselves from the parent skin 
(Hydra, medusae) or form cormi (Polyps, Siphonophora). 
Fission takes place either longitudinally, and is as such 
complete or incomplete (cormis), or transversely (stro- 
bila), and is as such always complete. 

General description of Ctenophora. The Ctenophora are 
Metazoa with a peisisting primary axis, and a modified 
radial structure. They possess an ectodermal oesophagus 
and a partly radial gastro-vascular apparatus, an apical 
sensory (nerve) plate, eight meridional rows of ciliated 
plates, in most cases a pair of tentacles and highly devel- 
oped muscular and connective tissues They are her- 
maphrodites. 

The Ctenophores are gelatinous, slightly colored, 



92 ZOOPHYTA OR CCEMNTKRATA. 

transparent animals, provided with two long tentacles 
and swimming freely in the ocean ; their general size 
varies between one centimeter and several decimeters. 
They resemble the medusae in form and habit, differing, 
however, very markedly by their peculiar mode of loco- 
motion which is carried on by eight meridional rows 
of conspicuous comb-shaped organs. The shape of a 
simple Ctenophore is that of an egg or pear, with the 
oral pole generally on the upper surface ; the opposite 
region is termed apical, or aboral or sensory pole. The 
structure of the body is quadriradial (each radius being 
forked), each radial plane containg two unequally dif- 
ferentiated interradei which are either medial, corres- 
ponding to the direction of the mouth slit, or transverse, 
lying in the direction of the tentacles. This radiation is 
conditioned by the position of the organs which may 
therefore be divided into radial (4 times repeated) and 
adradial (8 fold), medial (2 fold) and transverse (2 fold). 
Stereometrically the body can be divided into two sym- 
metrical halves, each one representing a symmetrical 
body. 

The external body epithelium is rich in pigment cells, 
irisated cells and glandular cells. Its differentiations 
are the contractile, richly pigmented tentacles, further 
eight rows of ciliated plates which proceed in four pairs 
towards the apical pole. The sensory plate lies in a con- 
siderable depression, an ectodermal thickening consist- 
ing of high narrow cells and probably representing the 
central nervous system. It is directly connected with 
the different sensory organs; from its surface radially 
•arranged compound sensory hairs arise carrying oto- 
liths (auditory orga?z); four pigment masses probably 
represent eyes and two ciliated polar fields joining the 



CTKNOPHORA. 93 

sensory plate in a median direction are generally consid- 
ered to be organs of smell. 

The month opening is a split lengthened in a median 
direction ; from it an ectodermal strongly ciliated oeso- 
phagus procedes leading into the transverse central 
stomach {infundibulum) of the gastro -vascular apparatus, 
from which the following peripheral canals radiate: 4 
radial ca?ials which fork and thus form 8 adradial meri- 
dional vesicles; two transverse oesophageal vessels disappear- 
ing near the mouth; two transverse tentacular vessels 
ending within the base of the tentacles; the infundibular 
canal, an axial continuation of the infundibulum open- 
ing near the sensory plate. 

The mesodermal gelatinous siibsiance contains connective 
tissue cells, mesenchymal muscle fibres and probably nerve 
fibres. 

The gonads are situated along the meridional vessels, 
ovaria along the one side and testes along the other; 
they are ejected through the gastro- vascular apparatus. 
(See Chun' s monograph on Eucharis). 

Their reproduction is exclusively sexual. Develop- 
ment is direct. The egg is small, but rich in yolk (cen- 
trally situated). Segmentation is unequal. The gas- 
trula is formed by a process which is both epibolic and 
embolic. At the blind end of the entodermal sac a cell 
plate is formed, which represents the unique structure of 
the mesoderm. (MetschnikofF). The oesophagus arises 
by evagination of an ectodermal tube on the vegetative 
pole: the protostome (as oesophageal opening) is thus low- 
ered. Four gastric pouches arise in the direction of the 
primary radii. In the surface eight groups of ciliated 
plates appear ; then the sensory plate at the apical pole, 
with the characteristic auditory apparatus, finally the 
tentacles remarkably near the apical pole. 



94 ZOOPHYTA OR CCEI,EN?£RATA. 

There are no other homologies between the three 
higher classes of the Ccelenterata than those which are 
the result of the common derivation from the gastraea, 
common to all Metazoa. 



PLATHELMINTHES. 



I Class: Turbellaria. Independent Plathelminthes 
with ciliated body-epithelium, living mostly on the bot- 
tom and under stones in ocean and rivers. 

i. Order: Polydadidea (pelagic forms). Large Turbel- 
laria with flat, leaf-like body, with numerous ovaria and 
testes, without yolk glands, mostly with two separate 
external sexual apertures. The alimentary canal sends 
out numerous branches which anastomose. 

i. Tribe: Cotylea. Ventral proboscis furnished with 
papillae. Mouth and pharynx central or anterior. Ten- 
tacles wanting or present on the anterior margin of the 
body. Anonymus. Thysanozoon. Yungia. Cyclo- 
porus. Stylostomum. Eurylepta. Prosthiostomum. 

2. Tribe: Acotylea. Without proboscis. Mouth and 
pharynx central or posterior. Tentacles wanting, or 
two present on the anterior dorsal region. Planocera. 
Leptoplana. Trigonoporus. Cestoplana. 

.2. Order: Tricladidea (pelagic, fresh water and terres- 
trial). Body flat and of considerable length. Mouth 
and protrusible pharynx posteriorly. With a common 
external sexual aperture with two germ stocks and 
numerous testes and yolk glands. The alimentary canal 
consists of an anterior, unpaired and two lateral pos- 
terior branches, which lead into side branches. Planaria. 
Dendrocoelum (fresh water). Geodesmus. Bipalium 
(terrestrial). Gunda segmentata (pelagic). 

3. Order: Rhabdoccelidea. In fresh water and marine. 



96 PtATHEjI.MIN^H^. 

Small forms. Alimentary canal, when distinctly visible, 
a cylindrical, straight, blind sac, either without any or 
with very few distinct lateral branches. 

i. Tribe: Alloioccela. Alimentary canal distinctly sep- 
arate from the parenchyma, frequently with short, lat- 
eral diverticula. Numerous testicular vesicles. Female 
germ glands either two ovaria or two germ yolk glands, 
or separate germ and yolk glands. Monotus. Plagios- 
toma. Vorticeros. 

2. Tribe: Rhabdocoela. Alimentary canal sharply sep- 
arated from the parenchyma, without lateral diverticula. 
Frequently large interstices arise in the parenchyma 
filled with a liquid substance and forming a kind of body 
cavity. Two large testes. Female germ glands, one or 
two ovaria or one or two germ stocks and yolk glands, 
or two germ yolk stocks. Vortex. Graffilla (parasitic). 
Macrorhynchus. Mesostoma. Prorhynchus. Micros- 
toma and Stenostoma (both with separate sexes). Mi- 
crostoma.' 

3. Tribe: Accela. Without distinct alimentary canal; 
with digestive parenchyma. Without excretory organs; 
with numerous very small testicular vesicles and two 
ovaria. Nadina. Convoluta. 

II Class: Trematoda (Flukes). Parasitic unseg- 
mented. Plathelminthes without cilia, mostly with 
forked alimentary canal. Mouth and pharynx at the 
anterior end of the body; two testes, one germ stock and 
two branching or numerously lobed yolk stocks. 

1. Order: Eetopar asitica (monogenetic Flukes). With 
at least three suckers. Development direct, without 
alternation of generation; life history simple, without 
heterogeny. Tristomum. Diplozoon (two young imma- 
ture animals early unite crosswise and in this condition 
become mature). Polystomum. Gyrodactylus. 



CESTODA. 97 

2. Order: Entoparasitica (digenetic Flukes). With 
two suckers at most. Life history with heterogeny. Dis- 
toma hepaticum, lanceolatum, both in the gall ducts of 
the sheep's liver. Distoma isostomum. Gynsecophorus 
haematobius, in the blood of Africans, both with sepa- 
rate sexes; the male with a groove, the canalis gynseco- 
phorus, on the ventral side for the reception of the female. 
Amphistomum Monostomum. The Sporocyst and Rediae 
generally live in water snails; the sexual generation 
mostly in the alimentary canal of the Vertebrates. 

Ill Class: Cestoda (tapeworms). Entoparasitic Plat- 
helminthes without cilia and without alimentary canal, 
with numerous testicular vesicles, two germ stocks and 
one or two lobed yolk stocks. Organs for attachment at 
the anterior extremity. 

i. Order: Monozoa. Unsegmented single persons. Am- 
philina. Caryophyllaeus. Archigetes. 

2. Order: Polyzoa. Colonies of Cestoda arising by 
strobilation: segemented tapeworms. With scolex and 
proglottides. Phyllobothrium. Tetrarhynchus. Lignla 
(indistinct external segmentation). Bothriocephalus 
latus: broad tapeworm in the alimentary canal of man. 
Over 3,000 proglottides With two sucking pits at the 
head. Sexual apertures in the middle of the ventral 
surface of the segment, one behind the other. Larva 
ciliated, aquatic. It becomes a Scolex, with six hooks, and 
lives in the flesh of the pike, the eel-pout, and perhaps 
of other fish. Schistocephalus. Triaephorus. Taenia; 
with four suckers. T. saginata (mediocanellata) without 
hooks at the rostellum, with over 1,000 proglottides, 
sexual apertures marginal; in the alimentary canal of 
man. The Cysticercoid lives in the muscles of cattle. 
T. solium; armed tapeworm of man, with double circle 



98 PI^ATHKlyMINTHKS. 

of hooks at the rostellum; sexual apertures marginal. 
About 800 proglottides. Cysticercus cellulosae in the 
flesh of swine. T. serrata in the intestine of the dog. 
Cysticercus pisiformis in the liver of rabbit and hare. T. 
crassicollis in the intestine of cats. Cysticercus fascio • 
laris in the liver of mice. T. cucumerina in the intestine 
of the dog; its cysticercoid scolex in the body of the dog- 
louse. T. ccenurus in the intestine of the dog. Its cor- 
responding cysticercoid Coenurus cerebralis in the brain 
and spinal cord of the sheep. T. echinococcus in the 
small intestine of the dog. Echinococcus veterinorum 
in the liver of man and different domesticated hoofed 
animals. 

General Description. — The Plathelminthes (or Platodes) 
are Scolecida (see below), without an anus; their body 
is dorso-ventrally flattened; their highly developed mes- 
enchyma produces the muscular cutaneous envelope, the 
dorso-ventral muscles and the muscles of the intestine, as 
well as the parenchymous connective tissue, which en- 
tirely or partly fills the primary body cavity; they possess 
a much-branched protonephridium (water vascular sys- 
tem); they are mostly hermaphrodites with complicated 
sexual organs. Of the three classes the Turbellaria 
represent the ancestral group; from them the Tremadota 
are derived, and from them again the Cestoda. 

The epithelium of the Turbellaria is a well developed 
vibratile epithelium, whilst that of the adult Tremadota 
and Cestoda, forming cuticular excretions, are hardly 
noticeable; subepithelial unicellular glands are generally 
present. The characteristic muscular cutaneous envelope 
of the Plathelminthes consists of an external continuous 
circular layer of 'muscles, of an internal laye? of longitudinal 
muscles, arrayed in bundles and of an innermost network 



CESTODA. 99 

of diagonal muscle fibres. The dorso- ventral muscles, be- 
tween the inner organs, anastomose at their ends. The 
parenchymals connective tissue, being largely vesicular 
(sometimes branched free cells), fills almost the entire 
primary bod}' cavity, but often more or less conspicuous 
interstices remain as remnants of the primary body cavity. 
We also find various muscles of the alimentary system. 
The nervous system consists of the cerebral ganglia and 
the peripheral nerves, and is in all parts imbedded in the 
parenchyma and in the muscular layers. The cerebral 
ganglion is not far removed from the point of origin (the 
anterior or vertex pole); it is mostly two-lobed or entirely 
separated in two parts, which are connected b}^ a trans- 
verse commissure (Trematoda). Numerous anterior 
?ierves continue towards the anterior termination of the 
body, whilst the paired longitudinal nerves extend pos- 
teriorly, the ventral pair being the strongest and most 
constant; the dorsal pair and the two lateral nerves are 
less frequent (Polycladidea, Accela, Tremadota). An 
oesophageal nervous system is present in the Dendroccela. 
The peripheral nerves are often connected by commis- 
sures (transverse commissure of the posterior ventral 
region; even a network occurs beneath the cutaneous 
muscular envelope. Ganglionic cells are mostly present 
in the cerebral ganglion; sometimes in the peripheral 
nerves. The Turbellaria have two, four or more pairs of 
eyes at the anterior end of the body; also an unpaired 
auditory vesicle, ciliated pits and tentacles in the same 
region. Among the ectoparasitic Trematoda and the 
larvae of the entoparasitic Tremadota occur cerebral eyes 
and organs of touch. To the Cestodes can only be ascribed 
a somewhat intensified sensitiveness of certain parts of the 
integment. Special sense-organs wanting. The aliment- 



IOO PI^ATHKlvMINTHES. 

ary canal of the Plathelminthes consists of the oesophagus 
and the stomach (intestine). The mouth performs also 
the functions of rectum and anus, which are wanting. 
The Cestoda feed endosmotically, so that the alimentary 
canal is degenerated; in the Acoela the digestive paren- 
chyma performs the function of the gastric system. The 
oesophagus is generally divided into two parts, an an- 
terior diverticulum and a posterior muscular pharynx, 
which can be protruded like a proboscis (Turbellaria), or 
serve as a sucking pump (Trematoda). Salivary glands 
are frequently imbedded in the muscular tissue of the 
pharynx. The stomach (intestine) shows the tendency 
to branch out (Dendrocooela, Trematoda) and to anasto- 
mose. Only that of the Rhabdocoela is a straight, blind 
sac, lying above the oesophagus. Its epithelium is mostly 
ciliated. The apparatus of excretion, generally termed 
water-vascular system, is distinguished by its numerous 
branches, which increase with the size of the forms; the 
excretory capillaries are very numerous and the collect- 
ing canals are more or less branched, showing even an- 
astomosis. Various modifications in arrangement and 
external opening occur already among the Turbellaria. 
A pair of longitudinal trunks, opening externally near 
the posterior end, represent the original type; they may 
also open more anteriorly on the ventral side (Mesostoma), 
or even at the posterior end; a median union of the main 
trunks also occurs. Among the Dendroccela numer- 
ous openings of the two (or more) main trunks on the 
dorsal surface have been observed. The excretory organs 
of the monogenetic Trematoda (Polystomum) generally 
open separately on the dorsal side, whilst the paired main 
excretory trunks of the digenetic Trematoda (Distoma) 
open into a posterior, unpaired urinary bladder. Among 



CKSTODA. IOI 

the Cestoda an increase of the main trunks is the rule; 
numerous secondary openings occur together with the 
main opening. 

The Plathelminthes are, with few exceptions (Micro- 
stoma, Distoma haematobium) hermaphrodites. Both 
sexual organs open in most cases into a common aper- 
ture on the ventral surface back of the mouth. The 
male apparatus consists of the' paired testes and Vasa 
deferentia, a dicctus ejaculatorius, prostate gland and an 
protrusible/^w. The female organ is more complicated 
since the egg which develops in it is of compound char- 
acter. The egg cell is derived from the (paired or un- 
paired) ovarium or germ stock, modifications of which 
develop into yolk stocks with numerous yolk cells, repres- 
enting a secondary nutritive material or abortive eggs. 
The ovarian ducts and yolk passages lead into the external 
passage (first into the ootype); here the egg is fertilized, *■• 
and together with a number of yolk cells enclosed by a 
chitinous secondary envelope which is secreted by special 
shell glands; the egg passage then terminates in the 
vagina, the organ of copulation. In most cases there is 
also present an uterus for the collection of the fertilized 
eggs and a receptaculum seminis, both being differentions 
of either oviduct or external egg passage or external 
sexual aperture. 

The modifications of the complicated sexual apparatus 
are multifarious and form an important and interesting 
chapter in the comparative anatomy of the Plathelmin- 
thes. The fundamental type of both sexual organs is 
represented (i) by two pairs of longitudinal canals, the 
gonad passages which contain in form of peripheral di- 
verticula the gonad (testes, ovaria) and (2) by the organs 
pf copulation (penis, vagina). The former possess a 



102 PLATHElvMINTHES. 

mesodermal epithelium, the latter an ectodermal epi- 
thelium. 

The eggs of the Plathelminthes are small, especially 
when a secondary nutritive material is present; segmen- 
tation is, therefore, equal, but the segmentation cavity 
is very small and gastrulation very probably epibolic 
(general among Scolecida). The whole animel has the 
general organization of a Plathelminth when it leaves 
the egg, but certain modifications occur. Only the 
larvae of the pelagic Planaria possess in their peroral cir- 
cle of cilia a larval organ which is probably palingenetic. 
The other Turbellaria, as well as the ectoparasitic 
Trematoda, undergo a direct development. The Distomae 
possess a so-called infusorial larva, which has the essen- 
tial organs of the Plathelminthes in rudimentary form. 
They are covered with large, ciliated cells (embryonic 
•envelope) which afterwards disappear. Also the Cestoda 
possess an embryonic envelope, either forming a ciliated 
cell layer (Bothriocephalus) or a shell- like apparatus 
(Taenia). The larva itself is very small and rudimen- 
tary. Both Distomae and Cestoda undergo a remarkable 
metamorphosis, whose individual stages must be looked 
upon as forms seco?zdarify acquired. This series undergoes 
further complications through asexual reproductive pro- 
cesses. Reproduction by fission occurs among all the 
Plathelminthes, parthenogenesis among the Trematoda. 

The distinction between the Ccelenterata and the Plathel- 
minthes may be based upon the general structure of the 
body cavity. The former constitute the one great sub- 
division of Metazoa with only one alimentary body 
cavity, whilst the latter, with all the other Metazoa, 
constitute the Ccelomata, which possess a series of body 
cavities. The transition from the Ctenophora to the 



CKSTODA. 103 

Turbellaria is represented by two forms: the Cceloplana 
Mecznikowi and the Ctenoplana Kowalevskii. They are 
not yet bilaterally symmetrical. They resemble the 
Ctenophora inasmuch as they possess an aboral sensory 
organ, eight rows of ciliated plates, tufted tentacles, 
and the same general structure; they are related to the 
Polycladidia by their flattened body, capable of crawling, 
their general ciliation, their skeletal membrane, their 
muscular fibres, the general arrangement of their gastric 
canals, two dorsal tentacles and a dorsal nerve centre (?) 
and a water vascular system (?). 

Hatschek includes the Plathelminthes (Platodes) un 
der his IV. phylum, the Zygoneura, containing the 
Scolecida, Articulata, Tentaculata and Mollusca, on the 
ground that they all possess a common ancestral larval 
form, the trochophora. The stage which ontogenetically 
precedes the trochophora is called the protrochula (no rec- 
tum). The Rotatoria always resemble the trochophora, 
whilst the Platodes only reach the protrochula stage 
(also called pilidium or scolex), whence they develop 
in a different direction from the other Zygoneura. This 
fact distinguishes them also from the other classes of the 
Scolecida. The Scolecida (see Hatschek's table) are 
Zygoneura with a prima^ body cavity, with mesenchy- 
mal muscles, with protonephridia and and with primary 
diverticular gonads. Their nervous system is mostly 
subepithelial. They are very small (often microscopic) 
animals of sluggish habit. 



VERMES. 



The branch of Vermes does not form a very homo- 
geneous or natural division of the animal kingdom. 
Its characteristics are, therefore, largely negative. All 
worms are bilaterally symmetrical animals of the most 
variable form. They surpass the preceding branches, 
inasmuch as they possess an anus and a blood-vascular 
system, which physiologically performs the functions of 
the gastro-vascular system. The absence of anus and 
blood system indicates secondary retrogression. The 
mouth opens on the ventral side of the anterior end of the 
body. A body cavity is either wanting or very variously 
developed. Beneath the external body epithelium there 
is in all shell-less forms a strong muscular layer {muscula? 
cutaneous envelope). The nervous system is likewise ot 
diversified structure. Only the presence of a nerve centre 
{brain, supra- oesophageal ganglion) above the oesophagus 
is constant. In most cases a nerve ring, encircling the 
oesophagus {oesophageal ring), occurs, from which longi- 
tudinal commissures proceed posterior^, different in 
length, position and arrangement. All these parts, brain, 
oesophageal ring and longitudinal commissures belong to 
the central nervous system. Segme?ited body appendages 
{extremities) are wanting as well as a^separate, muscular 
(ventrally situated) locomotory organ {foot). A strictly 
localized central organ of the blood system {heart) has 
only been observed among the Brachiopoda. 

I Ci^ASS : Nemertini (Rhynchoccela). Body ciliated, 



NEMATHKI.MIA. 105 

externally unsegmented, of elongated form, dorso-ven- 
trally flattened. Without a distinct body cavity, ali- 
mentary canal straight, mostty with lateral diverticula, 
anus at the posterior end of the body. Above the ali- 
mentary canal lies a separate protrusible proboscis, 
generally before and above the mouth. The central 
nervous system consists of a brain lying between pro- 
boscis and oesophagus and of two lateral commissures. 
Blood- vascular and excretory systems present. Sexes 
separate. Through regular repetition of internal organs 
(lateral digestive diverticula, circular commissures of the 
longitudinal nerves, sexual glands) a kind of internal 
segmentation (pseudometamerism) frequently arises. 
Almost exclusive pelagic. 

1. Order: Palceonemertini. Head without deep lateral 
longitudinal furrows. Proboscis without stylets. Mouth 
back of the brain. Carinella. Folia. 

2. Order: Schizonemertini. On either side of the head 
a deep, longitudinal split. Proboscis without stylets. 
Mouth back of the brain. Linens. Borlasia. Cerebra- 
tulus. Langia. 

3. Order: Hoplonemertini. Head without lateral fur- 
rows. Proboscis with one or more stylets. Mouth 
mostly before the brain. Amphipoms. Drepanophorus. 
Tetrastomma. Nemertes. 

4. Order: Malacobdellini. Head without lateral fur- 
rows. Proboscis without stylets. A sucking disk at 
the posterior end of the body. Malacobdella, parasitic in 
sea mussels. 

II Class: Nemathelmia (round worms). Body tu- 
bular, spindle or thread-like, unsegmented, covered with 
a thick cuticula. Body cavity generally spacious. Ali- 
mentary canal straight or wanting. Anus at the pos- 



106 VKRMKS. 

lerior end. No blood- vascular or excretory system re- 
sembling those of other worms. Sexes generally separate. 
Nervous system; an oesophageal ring, a medio-dorsal 
and medio- ventral longitudinal commissure. Internal 
metamerism wanting. Only the circular commissures 
of the longitudinal nerves repeat themselves pretty regu- 
larly among the Nematodes. Mostly parasitic. 

i. Order: Nematodes. With alimentary canal, with- 
out proboscis. Family Enoplidce without pharynx, fre- 
quently with eyes, live free in the ocean, more rarely in 
fresh waters or on land. Family: Anguillulidcz . Small 
animals (either parasitic or free) with double pharynx, 
without eyes. Tylenchus scandens (in wheat). Anguil- 
lula aceti in paste, vinegar, etc. Rhabditis nigrovenosa. 
In damp, muddy ground. Sexes separate. The females 
are viviparous, but produce only very few (4 at most) 
larvae which enter the lungs of frogs and toads and 
develop there into mature but hermaphroditic animals 
(Ascaris nigrovenosa), out of whose fertilized eggs the 
free living Rhabditis generation arises. Their life his- 
tory is characterized by a kind of heterogeny. Sphcer- 
ularia bombi. The Rhabditis larval form lives in the 
earth. The fertilized females enter the females of the 
bumble bee, in whose body cavity or alimentary canal 
they become parasites. The pregnant uterus evaginates 
from the genital opening (like a hernial protrusion) and 
becomes a large tube, on which the body of the worm 
forms only a small insignificant appendage. Mermithidce. 
Without anus. Larvae parasitic in the body cavity of 
insects, emigrate into damp ground, where they become 
mature and reproduce themselves. Mermis nigrescens. 
Filariidce. Filaria medinensis. Medina worm, 5-2 mm. 
thick, 1 meter long, in the tropical regions of the old 



NKMATHEI.MIA. 107 

world. In the subcutaneous connective tissue of man. 
Larvae in small crabs (Cyclopidae). Trichotrachelidce \ 
Trichocephahcs dispar, whipworm, posterior part of body 
swollen. In the human colon. Trichina spiralis. 
Mature as so-called intestinal Trichina in the small intes- 
tine of man, and of many mammals ; it is viviparous, 
female about 3 mm. long, male half as long. The 
young bore into the intestinal wall, and from here they 
penetrate (through the body cavity and the blood vessels) 
the muscle fibres,, where they encyst themselves in a 
calcareous cyst or capsule. Man receives them through 
infected pork. The chief carriers of the Trichina are 
the rats. 

Strongylid<z . Dochmius {Anchylo stomal) duodenalis, 
with a strong, mouth capsule, armed with teeth. Female 
2 cm., male half as long. In the small intestine of 
man (Egypt, Brazil, India, Switzerland, Italy, Belgium). 
Especially frequent among miners, producing miners' 
anaemia. Eustrongylus gigas. Female 30-100 cm. long. 
In the kidney of the dog and other mammals. Ascaridce. 
Ascaris lumbricoides (spool worm). Male 25 cm., female 
40 cm. long. In the small intestine of man. Oxyuris 
vermicularis . Female 1 cm., male half as long. In the 
large intestine of children. 

The Gordiidce occupy an isolated position among the 
Nematodes on account of their peculiar internal organi- 
zations. Mouth of adult closed, alimentary canal de- 
generated Gordius aquaticus. Mature form in fresh 
water. The embn^os enter the larvae of insects where 
they encyst themselves. If their host is eaten hy another 
insect they continue their development in the new host 
and when almost mature enter the water. 30-90 cm. 
long, 1 cm. thick. 



108 VKRMKS. 

2. Order: Acanthocephali. Mouth and alimentary canal 
wanting. At the posterior extremity a protrusible pro- 
boscis with hooks. Only parasites. Echinorhynchus 
gigas. In the small intestine of swine. L,arva in grubs. 

Ill Class : Annulata. Body long and tubular, or 
dorso-ventrally more or less flattened. Soft epidermis or 
hard and tough, chitinous cuticula. Metamerism or 
segmentation of the body, conspicuous both in the in- 
ternal organs and (mostly) also externally. Body cavity 
well developed (except in Hirudinea and Myzostomea). 
Blood-vascular system well developed, rarely entirely 
reduced. Alimentary canal is mostly straight from the 
mouth to the terminal anus. The nervous system con- 
sists of brain, oesophageal ring, and a segmented ventral 
ganglion chain. The system of excretion (wanting in 
the Myzostomea) consists of segmentally- arranged, paired 
nephridia. Frequently the nephridia perform the func- 
tion of emitting the genital products. 

i. Order: Hirudi7iei-Discophori (leeches). Body ex- 
ternally ringed; a certain number of external rings cor- 
respond to an internal segment. Around the mouth a 
sucker, beneath the arms a ventral sucker. Skin soft, 
setae wanting. Alimentary canal mostly with paired 
lateral diverticula. Body cavity reduced, communicat- 
ing with the well developed blood-vascular system. 
Numerous, segmentally arranged pairs of nephridia (loop 
canals), not used as sexual apertures. Hermaphrodites. 
Testes in several, segmentally arranged pairs, with 
special ducts, opening in one external sexual aperture. 
One pair of ovaria, situated before the testes ; female 
opening behind the male, both in the anterior part of 
the body. Parasites or robbers \ in fresh water, in the 
ocean and on land. 



ANNUL AT A. log 

1. Suborder. Rhynchobdellidcz (leeches with proboscis) 
Protrusible phara3'nx cylindrical, lying free in the 
pharyngeal pouch. Clepsine. Pontobdella. Branchellion 
(with gill-like appendages upon the back). The two 
last ones in the ocean upon Selachii. 

2. Suborder: G?iathobdellid<z. Pharynx a muscular 
thickening of the oesophageal wall, projecting towards 
the lumen in form of three plates or septa, sometimes 
toothed. Hirudo medicinalis, the common leech. H&mo- 
pis. Aulastomam, horseleech. Nephelis. Several Hiru- 
dines are terrestrial. All the other members of this sub- 
order live in fresh water. 

2. Order: Ch<ztopoda (worms with setae). An external 
segmentation mostly corresponds to an internal segmenta- 
tion. I* special segmentally arranged glandular sacs of 
the external skin free bristles (setae) arise, protruding 
above the skin. Body cavity well developed and separate 
from the blood-vascular system. The genital products 
develop in special districts of the entothelium of the 
body cavity; they enter early the body cavity and are 
emitted through modified nephridia (vasa deferentia, 
oviducts, genital tubes, segmental organs). 
The following division is purely artificial: 
1. Suborder: Oligochczta. With but few setae, which 
are never disposed on special parapodia. Tentacles, cirri, 
or branchiae wanting. Hermaphrodites. Direct develop- 
ment. In fresh water and terrestrial. Family: Aphano- 
neura. Acolosoma. Family: Naidomorpha. Nais. Dero. 
Stylaria. Family: ChcstogastridcE. Chcetogaster. Family: 
DiscodrilidcB. Posterior termination modified into a 
sucker. Parasitic in ciabs. Branchiobdella. Family: En- 
chytrcsidce. Pachydrilus. Enchytrcsus . Anachtzta. Family: 
Tuliftcidce. Tubifex. Psammoryctes, Clitellio. Limmo- 



IIO VERMES. 

drihts. Family: Phreoryctid<z. Phreoryctes. Family: Lum- 
briculus. Rhynchelmis. Stylodrilns. Family: Criodrilidce. 
Criodrilus. Family: Lumbricidce. Allurus. Dendro- 
bcEna. Allolobophora. Lumbricus (earthworm). Joining 
these Urochceta, Endrilus, Acanthodrilus, Perich<zta> 
Plenrochceta, Moniligaster. 

A doubtful position within the Chaetopoda occupy the 
so-called Archia,7?ielida {Polygordius, Protodrilns, Cteno- 
drilus, Histriobdelld) and Saccocirrus, forms whose organi- 
zation is distinguished by its simple, embryonic char- 
acter. 

Between the Oligochaeta and the Polychaeta are the 
families of the Capitellid<z (Capitellus, Notomastus, Dasy- 
branchus) and the Opheliacecs {Ophelia, Travisia, Polyo- 
phthalmus.') Blood vessels afe wanting in th^ former. 
The parapodia of both are much reduced. Gills present 
or wanting. Head not distinctly marked. 

3. Suborder: Polychtzta. Setae embedded in highly 
developed, segmentally arranged pads or humps or para- 
podia. Tentacles and tentacular cirri on the head; cirri, 
branchiae (gills) and other appendages along the ab- 
dominal segments, attached to the parapodia. Sexes 
separate in most cases. Development with metamor- 
posis. Marine forms. 

A. Sedentayia Capitibranchiata. Tubular worms. 
Pharynx (proboscis) generally not protrusible, without 
jaws. Byes wanting or small, but numerous on the head. 
Parapodia not well developed, the upper ones usually 
carry hair like setae; the lower ones transverse ridges 
with hooked setae or plates. Branchial (gills) mostly 
confined to the anterior segments or to the head. They 
dwell in tubes secreted or built up by the animal. 
'Family: Carratulidce. Cirratulus. Family: Arenicolidcz. 



ANNUIvATA. Ill 

Arenicola. Family: Spionidce. Family: Spio. Family: 
AriciadcB. Aricia. Family: Chlorcsmidcz. Siphonostoma. 
Family: Terebedidcz. t Lanice (Terebelld) . Polymnia. 
Amphitrite. Family: Serpulidcz. Serptda. Sabella. 
Spirographis. Myxicola. Protida. Family : Hermel-. 
tides, Sabellaria. Family: Stei'maspidce . Sternaspis. 

B. Errantia = Dorsobra?ichiata (predaceous worms). 
Pharynx protusible ; generally with jaws; head distinct, 
mostly with few, but large eyes. Parapodia well devel- 
oped. Branchiae generally on the dorsal parapodia. Free- 
swimming or crawling animals, some of which live in 
tubes made by themselves. Famih^: Aphrodites, Aphro- 
dite, HermiocE, Polynce. Family: Amphinomidez, Amphi- 
novie, Euphrosyne, Notopygos. Family: Eunicidtz, Dio- 
patra, Eunice, Hada. Family: Nereides, Nereis, Nep/i- 
thys. Family: Clyceridce, Glycera. Family : Sydidce, 
Haplosyllis, Sydis, Exogone, Aidolytus, Myrianida. 
Family : Hesto7iid<z, Hesione. Family : Phydodocidcz, 
Phydodoce. Family: Alciopidce, Alciope, Asterope. Fam- 
ily: TomopteridcE , Tomopteris. 

3. Sub-order: Echhiridce. Body tubular ; adult form 
unsegmented or indistinctly segmented ; without para- 
podia, cirri, and branchiae. Anteriorly on the ventral 
side two setae with hooks. Two anal glands (excretory 
organs) open into the terminal part of the much-twisted 
alimentary canal. Either two or three pair of nephridia 
or one nephridium. Anterior end of the body above the 
mouth lengthened into a long movable head lobe of 
various forms, with a ventral furrow. With a blood- 
vascular system. Sexes separate. Development with 
metamorphosis. Pelagic animals with a hidden mode of 
life. Echiurus. Thalascema. 



112 VKRMKS. 

Bonellia. The very minute, turbellarian-like, ciliated 
males of this genus live as parasites in the females. 

3. Order: Myzostomida. Body flat, disc-shaped, ex- 
ternally unsegmented. Margin of the body with cirri 
or short wart-like protuberances. Upon the ventral side 
five pairs of parapodia with hooks and supporting setae 
in two longitudinal rows. Four pairs of laterally-placed 
suckers on the ventral surface. Pharynx the same as 
that of the Rhynchobdellidse. Alimentary canal with 
lateral branches. Body cavity reduced. Organs of cir- 
culation, excretion and respiration wanting. The nervous 
system consists of the oesophageal ring and of a ventral 
cord united into a mass of ganglia. Brain reduced. 
Hermaphrodites. The oviducts open together with the 
alimentary canal in a cloaca. The seminal ducts open 
in two separate apertures on the ventral side. Hermaph- 
rodites, but in certain genera there exist small males be- 
sides (complemental males). Parasitic upon Crinoidea. 
Myzostoma. 

IVCivASS: Prosopygii. Body naked or within shells, 
of very varying form. Around the mouth a circle of 
ciliated tentacles or tufts which are often inserted upon 
a common horseshoe shaped tentacle carrier (lopho- 
phore), which itself may be extended on either side like 
arms. Without parapodia and frequently without setae. 
Anus almost in every case moved towards the anterior 
part of the body. The alimentary canal extends pos- 
teriorly and forms a loop turning again towards the an- 
terior end. Body not at all or very indistinctly seg- 
mented. Blood- vascular system wanting or differently 
developed. Number of nephridia reduced (two pairs at 
the most). They sometimes serve as conductor of the 
genital products, and open anteriorly not far from the 



PROSOPYGII. 113 

anus. Sexes separate. Only Phoronis is hermaphrodi- 
tic. Pelagic; onry few forms in fresh water. 

1. Order: Sipimculacea. Body lengthened, tubular 
naked. The anterior mostly extenuated part of the body 
may be invaginated by special retractors as a proboscis 
into the larger and longer posterior trunk. Body cavity 
very spacious. Blood- vascular system (?) much reduced 
or wanting. The central nervous system consists of 
brain, oesophageal ring and median ventral longitudinal 
commissure. Segmentation is perhaps indicated by a 
regular repetition of nerve rings. Marine; live in the 
mud or in concealment. 

1. Suborder: SipunculideB . Anus dorsal, situated an- 
teriorly between proboscis and trunk. Mouth surrounded 
by tentacles. Generally two typical nephridia, opening 
near the anus, also serving as eductory passages of the 
genital products. The vascular system consists largely 
of two tentacular vessels accompanying the anterior ali- 
mentary canal. Sipunculus. Phascolosoma. 

2. Suborder: Priapulidcs. Anus dorsal at the pos- 
terior end. No tentacles around the mouth. No blood- 
vascular system. No nephridia. Two anal glands 
opening in the immediate neighborhood of the anus and 
perform when young the functions of excretory organs, 
later those of sexual organs. Priapulus. At the pos- 
terior end a tuft of appendages, serving probably as' 
gills. Halicryptus without tail appendage. 

2. Order: Phoronidea. Body worm like, in a fastened 
chitinous tube. Numerous tentacles surround the mouth 
upon a horse shoe-shaped base. Anus dorsal, right next 
to the mouth. Around the mouth a nervous ring 
(oesophageal ring). Two nephridia opening anteriorly, 
serving at the same time as ducts of the genital organs. 



114 VERMES. 

A simple blood-vascular system present. Hermaphro- 
dites. Single germs: Phoronis. 

3. Order: Bryozoa. Small animals. Anus dorsal, 
near the mouth. A brain ganglion between mouth and 
anus. Nephridia when present, in one pair, embryonic 
in type, opening near the mouth, not serving as genital 
organs. Numerous tentacles upon a horseshoe-shaped 
base. They form mostly by budding sessile colonies of 
variable forms. 

1. Suborder: Pterobranchia. I^ophophores lengthened 
dorsally and posteriorly into a long, arm-shaped process, 
which carries two longitudinal rows of little tentacles. 
Alimentary canal confined to the anterior part of the 
body, lengthened posteriorly after the manner of a stalk. 
Body cavity little developed. Forming colonies, in 
tubes, which arise creeping upon a common stem. 
Rha bdopleu ra . Rel a ted : Cephalo discus. 

2. Suborder: Ectoprocta. Anus opening outside of the 
lophophore. IyOphophore not drawn out. Anterior body 
naked, posterior part within a shell. Without stalk. 
Anterior part enclosed in a fold of the posterior in such 
a way that it is wrapped in a special sheath (tentacle 
sheath), from which it can be protruded. Body cavity 
rather spacious. Shell often encrusted with calcareous 
matter. Forming colonies. A. Phylactolcemata. Lopho- 
phore horseshoe shaped. In fresh water. Cristatella. 
Alcyonella. Fredericella . Lophopus. Plumatella. B. 
Gymnolamata. Lophophores circular. Pelagic, with the 
exception of Pahidicella, Cellepora, Eschara, Bugula, 
Flustra, Akyonidium, Horner a, etc. 

3. Suborder: Entoprocta. Anus opening within the 
lophophore. A tentacular sheath wanting. Body stalked. 
With one pair of nephridia, Body cavity reduced, Pedi- 



PROSOPYGII. 115 

cellini forming colonies. Loxosoma single individuals. 
Pelagic. 

4. Order: Bi'achiopoda. The dorsal and ventral body 
wall forms two large (anterior) reduplications, so that 
the body is enveloped by a dorsal and ventral mantle 
lobe, which may unite posteriorly and laterally. The 
mantle lobes secrete a dorsal and ventral shell valve, 
which is mostly calcareous, but may be horny. The 
ventral one is generally more arched. At the sides of the 
mouth are inserted two long, buccal arms, covered with 
fine threads and rolled up spirally, often supported by a 
special calcareous skeleton of the ventral valve and lying 
within the mantle cavity, which is enclosed by the mantle 
lobes. Anus wanting or to the right of the mouth (only 
in Crania in the dorsal middle line of the posterior end). 
The central nervous system consists of an oesophageal 
ring with poorly developed brain and lower ganglia. 
One (rareW two) pair of nephridia, which serve also as 
ducts of the genital products opening into the mantle 
cavity to the right and the left of the mouth. Blood- 
vascular system probably present with a heart above the 
alimentary canal. The posterior end of the body fre- 
quently lengthens into a sessile stalk, which protrudes 
either between the shell valves (Lingula) or through a 
hole in a posterior elevation of the larger ventral valve. 
In man}' cases the stalk is wanting and the shell is fas- 
tened directly by the ventral valve. Exclusively pelagic. 
The large majority of genera and species fossil. The 
genus Lingula dates back to the palaeozoic epoch. 

1. Suborder: Testicai'dines . The valves are linked to- 
gether (lock) by processes like a hinge. Anus wanting. 
Terebratula. Waldheimia. Thecidium (grown fast with 
the large valve). Argiope. Rhynchonella, Spirifer. 



Il6 VKRMKS. 

2. Suborder: Ecardines. Without lock. Alimentary 
canal with anus. Crania. Lingula. 

V Class: Rotatoria (wheel animalcules). Small, 
mostly microscopic animals. Internal segmentation 
wanting. At the anterior end a ciliated organ (wheel 
organ) of variable form. Posterior end lengthened into 
an appendage (food, stalk), frequently segmented. A 
vascular system is wanting. A pair of nephridia of 
embryonic structure, with several internal ciliated cells 
open with the anus and the oviduct into the cloaca. 
Sexes separate. Males small with degenerate aliment- 
ary canal. Mostly in fresh water. Animals living in 
sessile tubes or envelopes : Floscularia. Stephanoceros. 
Melicerta. Laci?ndaria . The ciliated organ is thrown 
out in lobes and tentacles. The living forms : Notom- 
mata. Hydatina. Brachionus (external coat) Asplancha. 
Parasitic upon Nebalia in the ocean : Seison. 

The peculiar genus Dinophilus resembling certain 
annelid larvae belongs in this category. Male and female 
resemble each other, or the female is smaller, without 
alimentary canal. The whole ventral side of the body 
is ciliated. Besides there are a number of successive 
ciliated rings on the body. A wheel organ is wanting. 
Nephridia in segmental arrangement of embryonic type. 

Appendix. II Class : Chaetognatha (arrow worms). 
Body cylindrical, longitudinal, with laterally placed hor- 
izontal fins. Head rather distinct. Body cavity spa- 
cious, divided by partition walls into three succession 
cavities, head cavity, body cavity, caudal cavity. Mouth 
surrounded by setae (jaws). Alimentary canal straight, 
anus ventral, at the beginning of the tail. The central 
nervous system consists of the brain, the oesophageal 
commissures and a large ventral ganglion, No vascular 



CHAETOGNATHA. I I 7 

system. Hermaphrodites. Ovaria in the body cavity, 
testes in the caudal cavity. Paired eductory passages 
(nephridia?) open at the right and the left of the tail. 
Pelagic. Sagetta. Spadella. 

The small group of Gastrotricha may be placed near 
the Rotatoria, small animals with ciliated ventral sur- 
face, with longitudinal rows of bristles upon the back. 
Body ending in two lateral points. Alimentary canal 
straight, mouth followed by a muscular pharynx. Anus 
at the posterior end. Hermaphrodites. Nephridia 
insufficiently known. No blood system. Ichthydium. 
Mostly in fresh water. The Echinoderidce (related to 
Nematoda) are minute marine animals, with ringed body, 
with setae. 

General. The phylogeny of the worms is still a mat- 
ter of dispute. The Nemertini are, according to the 
latest investigations of Mcintosh, Semper, Hubrecht, and 
Burger, considered a separate natural class; the presence 
of a blood vascular system and an anus places them 
above the Plathelminthes. The systematic position of 
the Nemathelmia is very doubtful. It is probable that 
an acquired parasitic mode of life resulted in a degenera- 
tion. The Annulata constitute a very large group ex- 
traordinarily variable in form. The segmented condi- 
tion of the body seems to indicate the original type, 
whilst the Myzostomida, Echiuridse, certain Chaetopoda 
and, to a certain degree, also the Hirundinei may be re- 
trograded forms. Some investigators maintain that the 
segmentation (metamerism) of the Annulata is a further 
development of the pseudo-metamerism of Turbellarian 
or Nemertian animals. Others look upon the Annulata 
as a colony, risen through axial budding. They place the 
Rotatoria next to the ancestral type, whilst others con- 



Il8 VKRMKS. 

sider them to be simplified forms which mature in early 
developmental stages, thus remaining larval Annulata. 
The class of the Prosopygii consists of a number of dis- 
tinct natural orders, whose organization would be easily 
understood, if we base it upon an adaptation to the 
sessile mode of living and upon a reaction of a shell or 
tube formation upon the body of more highly developed, 
segmented worms. 

It may be necessary at this juncture, to say a few 
words on the structure of the Zygoneura larva Trocho- 
phora. Those characteristics which are present in the 
larvae of different groups are to be considered as typical. 
The trochophora is bilaterally symmetrical, exhibiting 
an anterior and a posterior, ventral and a dorsal part. 
The mouth- opening is ventral, the anus posterior and 
somewhat dorsal. Its form is oval. The distribution of 
cilia on the surface is characteristic. On the vertex pole 
a tuft of strong cilia {apical ciliary head) appears. An 
equatorial or preoral circle of cilia or trochus divides the 
body surface into an interior half (vertex field and a 
posterior half {opposite field). This circle lies closely 
before the mouth and consists of two rows of thickened 
epithelial cells. Behind the mouth there lies a postoral 
circle or cingidum (one row). Between these two the 
adoral zone of delicate cilia, is situated with its motion 
toward the mouth; from the mouth to the opposite pole a 
furrow of cilia, with posterior movements, extends (ve?itral 
furrow of cilia). A preanal circle frequently occurs, but 
is of later development. The ectoderm, furnishing the 
external epithelium as well as that of the oesophagus 
and of the hindgut, represents the body cavity (with 
spacious blastoccelom) in which the two other germ 
layers are contained. The external epithelium secretes a 



CHAETOGNATHA. 119 

distinct cuticula, consisting of supporting cells, ciliary 
cells, glandular cells and, as epithelial differentiations, 
the nervous system and the sensory organs, {primary cere- 
bral ganglion, vertex eyes, one or two apical tentacles, 
ciliary pits, and in some Turbellaria an apical auditory 
vesicle). The main typical nerve cords proceeding from 
the cerebral ganglion are a pair of longitudinal ve?itral 
nerves, a pair of longitudinal dorsal nerves, a pair of 
oesophageal nerves branching from the ventral nerves and 
forming a buccal ganglion in the epithelium of the oesoph- 
agus. The alimentary canal has the shape of a horse- 
shoe and consists of cesophagus (inward and forward, 
strongly ciliated, sometimes with chitinous appendage), 
midgut (entodermal shaped like a retort and divided 
into stomach with hepatic gland and small intesti?ie) and 
hindgut (leading to anus). The mesodermal structures 
lie between body wall and alimentary canal and are 
either mesenchymous (in the blastocceloni) or epithelial 
(forming special cavities). Mesenchymous are the co?i- 
nectivc tissue cells and the unicellular muscles, consisting 
of a pair of longitudinal ventral muscles a pair of longitudi- 
nal dorsal muscles, a preoral circular muscle and a postoral 
circular muscle. Muscles of the alimentary ca?ial are 
mostly small dilatators. The paired protonephridium is of 
mesodermal origin ; it is a long tube fastened to the pos- 
terior part of the vental muscle and anteriorly closed by 
a terminal cell. The tube itself consists of perforated 
cells and opens externally before the anus. A ccelom sac 
with ccelom cavities is present at the posterior end. 

Modifications of the trochophora depend upon the 
character of the metamorphosis of the animal. In the 
protochula or pilidium, the hindgut is not developed and 
the division of the midgut has not yet become distinct. 



120 VERMES. 

It is very probable that both the protrochula and the 
trochophora exhibit repetitions of the characters of an 
original ancestral form. It is true the Trochosphoera 
cEquatorialis (a Rotatorian animal discovered by Semper 
upon the rice fields of the Philippine islands) resembled 
the trochophora very closely; it nevertheless can be said 
that it is also a typical Rotatorian, and not a larval form. 
Hatschek, therefore, maintains that the protrochula is a 
repetition of the protochozoon, the ancestral type of all 
Zygoneura; but in any case the internal organization, 
(nervous system, alimentary canal, muscles, and protone- 
phridia) of every single group is to be explained in its 
relation to the ontogenetic stage of the trochophora. 
This is easily done in the groups of the Turbellaria, 
Rotatoria, and Bndoprocta; it is still a problem in that 
of the Nemathelminthes, whilst the structure of the 
Nemertini shows characteristics of the Aposcolecida or 
Cephalidii without, however, forming a transition. In 
the Aposcolecida new characteristics arise (peritoneal 
pouches, mesenteries, peritoneal gonads, metanephridia, 
blood-vascular system), whose ontogenetic origin is still 
a morphological problem. 

The body of the Nemertini is elongated, sometimes 
extremely so, reaching a length of several meters, while 
it is only a few millimeters thick. In their external 
ciliation and soft consistency they resemble the Turbel- 
laria. We distinguish the anterior part or head (char- 
acterized by the cerebral ganglion, the ciliated pits, the 
openings of mouth and proboscis), and the posterior 
part or trunk, in which certain organs (digestive diver- 
ticula, gonads, etc.) metamerically repeat themselves, 
although not always regularly and symmetrically. 

The lamination of the body is different in the different 



CHAETOGNATHA. 121 

orders. Palczonemertini exhibit the primary condition. 
The external epithelium is the seat of the various pig- 
mentations (supporting cells, glandular cells, sensory 
cells). Beneath it lies the dermal layer (connective sub- 
stance). This is followed by the somatic muscular 
structure, consisting of an external circular, and an inter- 
nal longitudinal layer, which is divided into septa by 
radial muscle fibres. The next, i. e., the parenchyma 
layer, connective substance with cell corpuscles, sepa- 
rates the preceding from an internal circular muscular 
layer, belonging to the alimentary canal, and therefore 
called splanchnic muscular layer. In the dorsal and 
ventral medium live the latter forms with the longitudi- 
nal muscular layer a kind of muscular dorsal and ventral 
mesentery. The splanchnic layer does not only enclose 
the epithelial tube of the digestive canal, but also the 
muscular sheath of the proboscis, within which the 
Rhynchoccelom and the proboscis are contained. The 
paired gonads lie in the parenchyma layer, the lateral 
blood-vessels, between the somatic and parenchyma layers, 
the dorsal vessel between alimentary canal and proboscis 
sheath. The central nervous system, terminating in the 
trunk in the form of lateral cords, lies always outside of 
the muscular layers. 

The lamination of the Hoplonemertini is almost the 
same as that of the preceding order ; however, the 
lateral nerve cords lie here, w 7 ithin the somatic muscular 
structure, and instead of the complete splanchnic muscu- 
lar structure dorso- ventral septal muscles arise between 
the gastric diverticula. 

In the Schizonemerti7ii a powerful external longitudi?ial 
muscular layer appears exteriorly and gradually into the 
so-called cutis (longit. muscles, connective tissue and 

G 



122 VERMES. 

subepithelial glands). The splanchnic layer is only repre- 
sented by septal muscles and the lateral cords always lie 
outside of the circular muscular layer. 

The protrusible proboscis, an invagination of the body 
wall, is a typical characteristic of the Nemertini. Whilst 
it is only short and little developed in the first and second 
order, in the third it reaches an enormous length and is 
divided into several regions, the first of which bears a 
number of styles, the second a poisonous gland and re- 
tractor muscles. Mouth and proboscis are one in Am- 
phiporus, Malacobdella and Geonemertes. 

The nervous system is highly developed and consists of 
the cerebral ganglion and the lateral cords, which run 
along the whole length of the body, often uniting beyond 
the anus. The cerebral ganglion consists of a pair of 
ventral ganglia, connected with the ventral cerebral com- 
missure, and a pair of dorsal ganglia, connected by the 
dorsal commissure, both embracing the opening of the 
proboscis; a posterior lobe (or ganglion in the third order) 
stands in closer relation to the cerebral ciliary pits. The 
lateral cords are provided with ganglionic cells, the whole 
structure resembling the ventral cord of the Annulata. 
From the brain nerves are distributed to the sensory 
organs, the oesophagus, the proboscis and throughout the 
whole length of the dorsum (a large and a small one). 
Metameral circular nerves proceed from the lateral cords 
connecting with the dorsal nerves; frequently a nerve 
layer, between the somatic muscles, takes their place. 
Eyes of inverse type are present in the head in various 
numbers; also paired auditory vesicles (CErstedtia). The 
cerebral ciliary pits have the function of organs of smell; 
they are typical for the Nemertini, being in them more 



CHAETOGNATHA. 1 23 

highly developed than in any other animal. Sensory 
papillae are found, especially in the head. 

The alimentary canal is straight and ciliated through- 
out its whole length. The mouth lies on the ventral side 
and leads into the simple oesophagus; the next part is the 
midgut (chylus), provided with lateral diverticula meta- 
merically arranged. A short hindgut opens directly into 
the anus. 

The blood- vascular system consists of an unpaired dorsal 
vessel and paired lateral vessels; often segmental trans- 
verse vessels and a vascular network of the head are 
present. The vessels are contractile. The blood contains 
large colored corpuscles. The apparatus of secretion con- 
sists of two lateral, short, ciliated canals in the region of 
the oesophagus, which open externall}'. Short, lateral 
branches of them are intimately connected with the lateral 
blood vessel. 

The Nemertina are rarely hermaphrodites. The paired 
gonads lie laterally within the parenchyma layer, and are 
metamerically repeated, alternating with the digestive 
diverticula. Each possesses its own eductory canal, 
which opens dorso-laterally and develops after sexual 
maturity is reached 

The small eggs (poor in yolk) are laid in large masses 
of spawn; some Nemertini are viviparous. The develop- 
ment occurs according to their different types: (1) De- 
velopment through the Pilidium larva [original type], 
(Linens, Nemertes). Equal segmentation. Blastula. 
Gastrula. Earva, morphologically corresponding to the 
protrochula, and termed pilidium, resembling a fencing 
mask. Two pairs of thickened ectoderm plates arise be- 
fore and behind the mouth, they invaginate and rep- 
resent the structure of the body wall with a third median 



124 VERMES. 

pair, which produces the ciliary pits. The connection 
between these embryonic plates and the body wall is 
called amnion. The plates grow together, forming a 
boat-shaped structure which surrounds the pilidium ali- 
mentary canal, and closes above it. Thus the young 
Nemertinus arises in the interior of the amnion cavity. 
The "body wall invaginates anteriorly, and forms the 
proboscis; the cerebral ganglion develops from a con- 
striction of the ectoderm. The old body wall of pilidium 
and amnion- envelope are cast off and the creeping 
Nemertinus is the result. The vertex plate disappears. 

(2) The development according to Desor's type [derived 
type], (Lineus), takes place within the spawn before 
leaving the egg membrane; a distinct pilidium larva is 
not present, but even here the ectoderm separates into 
an external ciliary embryonic membrane and a number 
of embryonic discs, which invaginate and form the 
Nemertinus. (No amnion.) 

(3) Direct development (modified type), takes place 
when the young Nemertinus arises from entire cell 
material. But even here the expulsion of an external 
ciliary layer of cells has been observed. 

In all cases the young Nemertinus possesses only a 
very short abdomen, only later growths enlarge it con- 
siderably. 

Hatschek assigns the Nemertini a middle position be- 
tween the Scolecida and Aposcolecida preceding them by 
the Gastrotricha, Rotatoria, Entoprocta, and Nemathel- 
minths. 

The unsegmented body of the Nematodes is rounded, 
more or less elongated, and both ends are, as a rule, 
tapered off. The mouth opening lies at the anterior end, 
the anus mostly at the posterior or ventral side. Many 



CHAETOGNATHA. 1 25 

Nematodes are almost microscopically small, others may 
become as large as a large earthworm. Ciliation is ab- 
sent in the whole life history of the group. A powerful, 
chitinous, transparent cuticula (which is periodically 
shed) covers the whole body ; it is often transversely 
ringed externally whilst the internal layer consists of 
diagonally intersected fibrous systems. Spines and 
hooks may also be present. The hypodermis or subcuti- , 
cula lies beneath the cuticula ; it consists of fibrously 
granular tissue in which mudei are embedded. The 
two lateral lines of the body appear to be powerful thick- 
enings of the subcuticula along which (at least anteriorly) 
the excretionary canals are situated. The so-called dor- 
sal and ventral median lines, however, are narrow, 
septa-like, projecting (internally) thickenings of the 
subcuticula within which several nerve cylinders are 
located. Immediately beneath the subcuticula lies a 
muscular layer, the innermost stratum of the body wall, 
consisting of longitudinal muscles (two dorsal and two 
ventral) arranged in form of four similar muscle fields 
and extending along the interspaces of the four longitu- 
dinal lines throughout the whole body. The histology 
of the muscular structure is very remarkable, dividing 
the Nematodes into Meromyaria, Platymyaria, Polymy- 
aria and Coelomyaria, according to the number and ar- 
rangement of the muscle cells. The body wall encloses 
a distinct body cavity with alimentary canal and sexual 
organs. An undifferentiated connective substance with- 
out cells, but distinctly arranged, fills the space between 
body wall and entrails. A cross section of the body 
shows an external epidermis, an epithelial muscular 
layer and the alimentary layer. The alimentary canal 
(straight and elongated) is divided into oesophagus, mid- 



126 VERMKS. 

gut and hindgut (proctodeum). A mouth with lips 
leads into a buccal cavity lined with a firm dentary cuti- 
cula; lips and teeth form distinct systematic characteris- 
tics. The oesophagus consists of an external simple 
membrane [propria], thick muscular walls, a chiti- 
nous lining and a triangular lumen, and is frequently 
dilated behind into a muscular bulb (pharynx), the chi- 
tinous lining of which is sometimes raised into ridges or 
tooth-like prominences to which the radial muscles 
converge in the form of conical bundles. Instead of 
the bulbus a gland of large dimensions may arise. 
The chylus consists of basal membrane, epithelial 
layer and internal, perforated cuticula. The hindgut is 
formed by an internal cuticula, epithelial layer and ex- 
ternal muscular layer. Several unicellular glands open 
near the ventral (rarely terminal) anus. The excretory 
canals lie close to the lateral lines in the anterior part of 
the body, changing into short transverse canals near the 
oesophageal ring and opening ventrally into one aperture. 
The central nervous system consists of a ring surrounding 
the oesophagus. Anteriorly it sends offsix nerve trunks, 
two along the lateral lines, and four submedian, which 
supply the papillary sense organs around the mouth, and 
are connected with numerous ganglion cells behind the 
lips. Posteriorly four submedian nerves are sent off, 
but they can only be followed a certain distance, whilst 
a dorsal and a ventral nerve extends the whole length of 
the body, corresponding to the ventral and dorsal muscle 
spaces, and here and there connected by ring fibres. 
The large Ascarides possess even an anal ganglion, some- 
times a complete anal ring connected with posterior 
nerves and ganglion cells. The sensory organs are con- 
stituted by the oral and posterior papillae of touch, a.u 



CHAKTOGNATHA. 1 27 

eye spot near the oesophagus (only In free Nematodes) 
and bristles of touch. The sexes of the Nematodes are 
separate (only few exceptions and one case of heterogeny). 
The males are smaller than the females, their ventral 
posterior termination being bent and covered with papillae 
of touch. The female organs are coiled canals, the 
blind end of each of which forms the ovarium, and the 
successive parts the oviduct and uterus, leading to a 
short vagina. The different parts of the unpaired male 
genital canal are the testes (amoeboid spermatozoa) the 
vas deferens and the bladder ; it opens ventrally in the 
hindgut (cloaca). Paired chitinous evaginable spicula, 
situated in special pouches on the dorsal side of the 
hindgut form the apparatus of copulation. The termina- 
tion of the cloaca itself may be protrusible ; a bursa 
copulatrix is sometimes present. Development is direct. 
The oval fertilized eggs, provided with a rich yolk mem- 
brane, accumulate in the uterus, either sparingly or in 
large numbers. Development begins either after the sep- 
aration of the eggs or already in the uterus. Segmenta- 
tion is equal, but without a segmentation cavity; gastru- 
lationby epibolism, gastrula mouth corresponding to the 
ventral line; two primitive mesodermal cells separate and 
furnish two mesodermal streaks, which separate into the 
four muscle regions, the excretory canals and the genital 
structure, the latter represented by two embryonic cells, 
typical in young Nematodes. Stomodaeum and Procto- 
daeum arise as ectodermal invaginations. The embryo is 
then oval, lengthens and lies finally spirally coiled in the 
egg membrane. The young animal shows already the 
typical characteristics of the Nematodes. They are often 
supplied with special arrangements for their parasitic 
wanderings 



128 VKRMKS. 

The habitat of the Nematodes is very curious. The 
free individuals live in decaying matter, mostly in the 
ocean. Parasitic forms change their host at different 
stages of their development. 

The Gordiidce occupy a somewhat isolated position. 
Lateral lines are wanting. A cerebral ganglion, oeso- 
phageal ring and ventral cord seem to be present. The 
paired genital sacs extend through the whole length of 
the body, and are so intimately grown together w r ith the 
longitudinal layer of the muscles that they resemble the 
peristoneal sacs of the Ccelomata. The genital organs 
open in both sexes by special posterior apertures. The 
terminal part of the male is forked and supplied with 
papillae. 

The Acantocephali are Scolecida (?) of entoparasitic 
habit; the alimentary canal is wanting; without cilia in 
all stages of life; the elongated, round body carries an- 
teriorly an invaginable proboscis provided with hooks, 
behind which a ganglion is located; the body cavity con- 
sists of a thin cuticula, a powerful subcuticula with a 
system of subcutaneous canals, an external circular and 
an internal, longitudinal, muscular layer. Paired, club- 
shaped cumulations of the. body, wall (lemnisci) pro- 
trude anteriorly into the spacious body cavity; sexes 
separate, the complicated genital organs open at the 
posterior end, development with peculiar metamorphosis. 

The phylogeny of this group is greatly disputed. The 
tender cuticula and thread-shaped spermatozoa are pecu- 
liarities which make their relation to the Nematodes 
very doubtful. Their peculiar organization has evi- 
dently been acquired by adaptation to parasitic habits; 
it may also be probably an account of their remarkable 
histological structure that they are derived from smaller 



CHABTOGNATHA. 1 29 

forms and that parasitism resulted in a considerable in- 
crease in size. 

Most of the Acanthocephali are only one or a few 
centimeters long; Echinochynchus gigas, however, may 
reach a length of 50 cm. There is little difference be- 
tween dorsal and ventral side. The chitinous recurved 
hooks of the proboscis with which they fasten them- 
selves in the alimentar}^ wall of their host, are derived 
together with the cuticula from the subcuticula. An 
internal retractory muscle and a sheath with lateral 
retractor} 7 muscles constitute the mechanism of the 
proboscis. The ganglioji at the base of the sheath 
sends nerves to the genital organs and the lateral body 
wall. The Subcuticula consists of vertical fibres and nu- 
merous granules, and a vascular system of subcutaneous 
canals filled with a granular fluid. Both muscular layers 
are composed of immense muscular cells of remarkable 
structure interwoven by the general connective sub- 
stance. The lemnisci are continuations of the subcuticula 
and of the same structure. They probably serve the 
purpose of transferring the food to the body cavity. The 
male and female sexual organs are structurally of similar 
t}'pe. They are imbedded in an axial ligament (mostly 
undifferentiated connective substance), which extends 
from the sheath of the proboscis to the posterior end of the 
body. The male apparatus consists anteriorly of £ pair 
of testes (with the thread-shaped spermatoza), the vasa 
deferentia, ductus ejaculatorius (with prostata), penis and 
protrusible bursa copulatrix. The ovarium of the female 
apparatus separates into cell balls; isolated maturing ova 
float in the body cavity. The eductory apparatus begins 
with a funnel-shaped organ (uterus bell, composed of a 
few immense cells), whose anterior large opening re- 
G* 



130 VERMES. 

ceives (swallows) masses of eggs. Premature eggs are 
ejected into the body cavity by posterior dorsal openings; 
the mature, spindle-shaped eggs are carried through pos- 
terior, narrow canals (egg passages) into the oviduct 
(uterus) and from there into the vagina. Development 
takes place in two periods, embryonic development and 
metamorphosis. The latter is connected with a typical 
change of hosts. The sexual animals live in the alimen- 
tary canal of certain vertebrates (fish). The eggs with 
the ripe embryo are thrown out with the excrements and 
eaten by Arthropoda, where the larva bores through the 
alimentary canal and enters the body cavity. After de- 
veloping into the young adult it returns to the old host 
and becomes mature. 

The Gastrotricha are Scolecida with paired ventral 
ciliary band ; mostly with forked tail appendage ; with 
terminal mouth opening and oesophagus (similar to Ne- 
matoda). 

We mention this group here, because the latest inves- 
tigations of Zelinka show that they constitute a special 
class of the Scolecida. The oral ciliary circles (of the 
Rotatoria) are wanting ; the mouth-opening is terminal, 
and the vertex plate dorsal. But other characteristics of 
the Trochophora, wanting in the Rotatoria, are present 
here, e. g., the ventral ciliated band and the ciliary pits. 
Hatschek places them immediately after the Plathel- 
minthes. 

The Gastrotricha are fresh water animals of micro- 
scopic size. Their form is spindle-shaped, the ven- 
tral side being flattened. The body is covered with a 
firm cuticula, which undergoes scaly differentiations. 
The cerebral ganglion is intimately connected with the 
anterior epithelium and its sensory cells and is situated 



CHAETOGNATHA. 1 3 1 

dorsally before the oesophagus ; it sends out posteriorly 
two ventral longitudinal nerves. The muscular system 
is represented by paired lateral and ventral longitudinal 
muscles. The excretory organs consist on either side 
of a single coiled tube with vibratile terminal apparatus 
and open in the middle of the body. The straight ali- 
mentary canal lies in a distinct body cavity. The 
mouth opening (with a circle of bristles) leads into 
an oesophagus (with triangular lumen and radial mus- 
cles) which is followed by a midgut consisting of large 
cells. A short hindgut opens above the tail appendage. 
It is not established whether they are hermaphrodites or 
of separate sex, since the existence of male organs has 
not yet been proved. The ovarium (posterior and ven- 
tral) consists of an accumulation of small cells which 
grow into very large eggs with a tough, often prickly 
yolk membrane. They are laid before development 
begins. This is direct, the young possessing the charac- 
teristic organization. 

The Rotatoria logically follow the Gastrotricha. In 
their whole organization they stand nearest the trocho- 
phora type, which of course, is more or less modified. 
Essentially new characters are the separation of the ner- 
vous system from the epithelium, the dilated masticatory 
pharynx and the cloaca. 

It is usually possible to distinguish between an anter- 
ior region (with cilia) of the body, which can be invagi- 
nated into the middle region whose cuticula forms a more 
or less rigid armor (sometimes externally segmented). 
The tail-like movable ventral foot appendage (mostly 
with two rami into which glands open) may also be 
retracted into the armor. 

The anus is situated on the dorsal side of the foot. 



132 VERMES. 

In sessile Rotataria the foot becomes a long contractile 
stalk which changes the position of the anus ; in some 
cases the foot is entirely wanting. The simplest form 
is the Trochosphsera sequatorialis whose wheel appa- 
ratus is not retractile and whose trochus extends along 
the equator of the spherical body. There are also 
remarkable complications of the body form arising 
through the development of external movable appen- 
dages of the middle region. 

The form of the body is also modified by the various 
differentiations of the ciliary circles (wheel apparatus). 
The trochus has locomotory, the*cingulum prehensile 
functions. They may be lobed or disappear altogether ; 
they may either surround the whole vertex field (flat or 
concave), or they may be interrupted in the dorsal and 
ventral median line. 

The^ body cavity consists of the external cuticula 
(more vigorous in the middle region) and of the hypo- 
dermis, consisting of thin flattened cells. The muscles 
of the body wall do not form a connected layer ; most 
constant are a few unicellular longitudinal retractory 
muscles extending through the body. The spacious 
blastocoelom contains the entrails kept in position by 
connective tissue cells and muscular cells. Special 
cells of the hypodermis are more powerfully devel- 
oped (ciliary, senso^, glandular cells). The aliment- 
ary canal (straight or horseshoe-shaped) shows char- 
acteristic complications. The month- opening leads into 
the foregut (stomodaeum) which is divided into the cili- 
ated buccal cavity and the masticatory pharynx- with 
chitinous masticatory apparatus, consisting, in most 
cases, of a pair of external and a pair of internal maxil- 
lae, important for classification. The midgut (entoder- 



CHAETOGNATHA. 1 33 

mal) is divided into the membranous ciliated oesophagus 
with the midgut glands or pancreas, the stomach com- 
posed of large, ciliated epithelial cells and the narrow, 
ciliated hindgut opening into the cloaca (not ciliated). 
The excretory apparatus (water vascular) is atypical proto- 
nephridium, consisting of glandular, longitudinal canals 
extending to the anterior region and forming lateral 
branches. They open either by means of a contractile 
vesicle or directly into the cloaca, when the bladder is 
an independent appendage. The central nervous system 
consists of a cerebral ganglion, situated dorsally upon 
the oesophagus. Among the sensory organs the eye 
spots are most prominent, having mostly the shape of 
an X. Cuticular sensory organs, consisting of groups of 
sensory cells, provided with a tuft of sensory hairs are 
present on the vertex field (apical tentacles?) in the 
middle region (dorsal and ventral pair), and on the 
trochus. The peripheral nervous system consists of 
nerves of the anterior region (to the frontal tentacles and 
the cells of the ciliary circles) and of longitudinal nerves 
(dorsal and ventral pair to the tentacles or blending into 
one). From these the nerves of the muscles branch out. 
The sexes are separate. The unpaired ovarium on 
the ventral side of the female alimentary canal contain 
beside the germ cells a mass of large nutritive cells. A 
short oviduct (uterus) opens into the cloaca. The males 
rarely appear and possess a rudimentary organization. 
Most Rotatoria reproduce themselves throughout the 
largest part of the year, parthenogenetically, by means 
of so called summer eggs (the smaller enveloping the 
males). The hard -shelled winter eggs are, perhaps, not 
always developed. Development mostly direct, either 
after the eg^ has been deposited or in the oviduct. Seg- 



134 VERMES. 

mentation of the small eggs (poor in yolk) is equal or 
slightly unequal (of the oligomerous type); grastulation 
epibolic. Habits modified, by adaptation. Hatschek 
divides them into Vagantia (free swimming or crawling) 
and Tubicola (sessile, mostly with gelatinous capsule). 
Following Hatschek's interpretation of the Vermes, 
we think it best to introduce the Entoprocta at this junc- 
ture. They are classified by Lang as a suborder of the 
Bryozoa, because they produce buds, which either be- 
come free or — and this is of importance here — remain 
united as a corm or colony. There are also other pecu- 
liarities in their organization which ally them to the 
Bryozoa. The chief reason, however, why they ought 
to be separated is based upon their entirely different 
phylogenetic derivation. The larva of the Entoprocta 
possesses an equatorial circle of cilia, which may be 
compared with the trochus. The convex region of the 
body (vertex) possesses at the apical pole a structure 
surrounded by sensory hairs and further ventrally a re- 
tractile ciliated structure; one of the two might corres- 
pond to the vertex plate. The opposite ciliated region 
contains the opening of the mouth, of the anus, and of 
the simple protonephridium in typical position. The 
mouth opening leads into a horseshoe-shaped, ciliated, 
alimentary canal, consisting of the typical ectodermal 
oesophagus, the stomach with a so-called liver, intestine 
and ectodermal rectum. The body cavity is primary. 
The larva is able to invaginate the opposite region, 
which assumes the form of an atrium with ciliary circle. 
Thus the larval organization fully corresponds to that of 
the adult, excepting the ciliated circle, which in the lat- 
ler consists of ciliated tentacles, and the stalk-like elon- 
gation of the vertex region, by which the adult is fast- 



CHAKTOGNATHA. 1 35 

ened. A vertex ganglion is wanting in the latter; but 
one is present between mouth and anus, sending nerves 
to the tentacles and to a pair of organs of touch. The 
Entoprocta are hermaphrodites or of separate sex. The 
paired testes and ovaria above the stomach are sac-like 
gonads opening before the anus. Loxosoma forms buds 
on the external wall of the cup, which separate as free 
individuals. In Pedicellina and Urnatella they arise on 
the stolo at the base of the stalk successively and form 
a cormus. 

Before entering upon a discussion of the Annulata it 
will be necessary to establish the comiection between the 
Scolecida a?id the Aposcolecida or Cephalidia. In the de- 
velopment of the latter the primary or trochophora organs 
occur (Scolecida stage), but they are confined to an ante- 
rior region of the body, the prosoma. Some of these 
organs remain larval organs and retrograde (protoneph- 
ridium or prosoma kidney). In a posterior region, the 
metasoma, a number of secondary organs are developed, 
mostly differentiations of the'ccelum sacs. They are the 
peritoneal epithelium surrounding the ccelum cavity, the 
germepithelium of the gonads, the metanephridia, paired 
tubes connected at one end with the ccelum and opening 
externally at the other end. They serve not only as 
kidneys, but also as genital ducts. Their morphologi- 
cal origin has not yet been fully established. Epitheli- 
ogenous muscles, derived from the ccelum epithelium, are 
especially the longitudinal muscles of the body. The 
external muscular layers, probably derived from the 
primary muscles of the Scolecida, are mesenchymatous 
structures genetically connected with the ccelom sacs. 
The blood-vascular system — typical for the Cephalidia — 
appears originally as a closed system of tubes in which 



136 VERMES. 

the blood is circulated by means of the contractions of 
the vessels. Its typical parts are a dorsal vessel (interior 
circulation) and a ventral vessel (posterior circulation) 
both connected by a network of visceral vessels and 
somatic transverse vessels. In the higher types the 
dorsal vessels are changed into a heart. The ventral nerve 
cord (absent in Phoronis and the Bryozoa) derived from 
the ectoderm, is a continuation of the ventral longitudi- 
nal nerves, which form [separated] in the prosoma the 
oesophageal commissures, but are in the metasoma more 
closely connected by transverse commissures, and thus 
form the ventral cord. 

Whilst the prosoma originally forms the larger part of 
the body, later developments enlarge the metasoma to 
such a degree that it far excels the prosoma in size, and 
its ccelomatic organs extend secondarily into the latter, 
where a retrogression of the protonephridia and the pri- 
mary longitudinal muscles has taken place (Annulata). 
In all Articulata (Hatschek) a multiplication of the meta- 
soma takes place, a repetition in the long axis, introduced 
by a repetition of the coelom sacs (primitive segments), 
and termed metamerism. Prosop}^gii and Mollusca are 
unsegmented. In every somite or metamere all the organs 
are repeated in the same order and arrangement, so that 
each one presents an anterior and a posterior end. The 
metameres arise successively by the continuous division 
of the undifferentiated terminal segment whose posterior 
portion finally remains as the unsegmented or rudimen- 
tary termination. The prosoma of the Articulata differs in 
many respects from the metameres. Their body is, there- 
fore, divided into, (1) the prosoma with prostomium and 
metastomium; (2) the metasoma, consisting of a number 
of metameres, whose last segment mostly carries the 



CHAETOGNATHA. 137 

periproct (anus). Homonomous metamerism (metameres 
all alike) occurs only in lowest forms; the division of 
labor requires a differentation (Jieteronomous metamerism), 
which may refer only to the internal organization or to 
the external, which latter is.commonly meant by the term 
heteronomy. Generally groups of metamers are modified 
in the same way and cause the formation of regions, as 
in the case of the Insecta, Crustacea and Myriopoda. 
The fotmation of the head is very irregular in the Articu- 
lata. It occurs gradually through the modification of 
the first metameres. Only in Protodrilus and Polygardius 
all metameres are alike. The prosoma alone is distinct 
from the metameres. It, therefore, constitutes the pri- 
mary head of the Articulata. In all the Annulata a simi- 
lar condition is developed. In the Arthropoda for the 
first time a distinct head is developed, and may be desig- 
nated as typical for the branch. The metameres of the 
head become organs of the mouth. The head of the 
Crustacea consists of the prosoma and four metameres; 
so also that of the Tracheata (?). 

The metameres arise from the primitive structures of 
the simple undifferentiated metasoma by a more distinct 
separation of certain parts. These primitive structures 
are: (i) the external epithelium or ectoderm; (2) the 
paired ccelom sacs which may also be represented by 
solid mesodermal streaks ; (3) the digestive epithelium 
or entoderm; and (4) the mesenchymous cells situated 
between these lamina. Division (most characteristic) 
appears in the mesodermal streaks or ccelom sacs which 
separate into a number of successive sacs or hollow plates 
(primitive segments) ; later the segmentation of the ecto- 
derm begins with the development of its organs and seg- 
mented constrictions ; the entodermal segmentation occurs 



138 VKRMBS. 

last and is rather indistinct. In many Annulata the 
formation of metameres begins in the free swimming 
trochophora larva and is, in the lowest forms, continued 
even after the sexual organs have become mature. 
Growth takes place at the posterior end. The full num- 
ber of segments may be present already in the embryonic 
development (Oligochseta, Hirudinei), or at the end of 
the larval stage or at the time of sexual maturity. In 
many Crustacea at least two segments arise already dur- 
ing the larval stage, the others in the course of meta- 
morphosis. In the higher Crustacea metamorphosis is 
suppressed and the metameres are formed during em- 
bryonic life. The same is true of the Insecta where all 
the segments appear at the same time. The Chilognatha 
increase their segments after they have left the egg 
membrane. 

Metamerism also occurs among the Vertebrates and 
the Echinodermata (in each of the five radii of the body). 
The former is genetically derived from the Articulata ; 
the latter may probably be traced to a special develop- 
mental stage. The Prosopygii and Mollusca are very 
l ; kely derived from polvmetamerous forms, in which a 
reduction to one single metamere took place, capable of 
a much higher development. 

The Annulata are characterized by their homonomous 
segmentation (without typical compound head). Many 
investigators consider the Annulata the phylogenetic 
starting-point for all the succeeding higher animals. On 
that account they are morphologically of supreme inter- 
est. Hatschek maintains that Protodrilus and Polygordius 
stand nearest the original ancestral form of the Annulata, 
and therefore classifies them as Archiamielides. From 
these simplest forms the genealogical scale leads' up to 



CHAETOGNATHA. 1 39 

the Chsetopoda, highest among which the Rapacia 
(Errantia) rank. The Hirudinei and Pligochaeta are 
a secondarily simplified and modified form, so likewise 
the Kchinridse. 

The Archiannelides are small Annelides (Annulata) of 
very primitive organization ; with large, distinct meta- 
stomium; with perfect external and internal homonomy 
of the metameres; with long primary tentacles (spioni- 
dous); with epithelial nervous system; ventral cord 
unsegmented; without bristles, without cirri, without 
parapodia ; development with larval metamorphosis. 

The genus Protodrilus represents the ancestral type of 
the Annelides in its simplest form. The body is elon- 
gated and round, growing thinner towards the posterior 
end. Metajnerism as given above. All segments possess 
gonads; they constantly multipy; the terminal segment 
contains the anus and two glandular pointed processes. A 
ventral ciliated furrow of locomotory function extends from 
mouth to anus. There are besides a great many circles 
of cilia (preoral, postoral and segmental). The epithelial 
layer of the body consists of an external cuticula and 
the epithelium, containing supporting cells, glandular 
cells, sensory cells and nervous tissue. The ventral cord 
is divided into two divisions, running along either side 
of the ciliated furrow. The cutaneous muscular envelope 
consists of the longitudinal muscular la3 T er (two dorsal 
and two ventral regions), whose fibres are arranged like 
the leaves of a book, the nuclei lying along the inner 
edge. A circular muscular layer is wanting (simplifica- 
tion of original character). The somatopleura lines the 
body cavity, which is not only divided into a right and 
left half by a dorso ventral mesentery, but also into two 
chambers by transverse septa (composed of the transverse 



140 VERMES. 

muscles and the peritoneal epithelium). The latter forms 
the germ epithelium on the side turned towards the chief 
chamber. The alimentary canal (splanchnopleura, mus- 
cular layer, epithelial layer), beginning with the mouth 
opening, consists of a short oesophagus, with glands and 
peculiar pharynx, and of the chylus, a straight, ciliated 
tube. The blood-vascular system is composed of a sinus 
surrounding the chylus and growing behind the oesopha- 
gus into a contractile, heartlike dorsal vessel, which forks 
anteriorly. The two branches pass on to the tentacles, 
oesophageal commissures and ventral line, where they 
unite. The segmental organs (excretory, retroperitoneal) 
are present in all metameres. They consist of the ciliary 
infundibulum (opening in the preceding segment), and 
the canal running along the lateral line and opening exter- 
nally. The ce?itral nervous system is composed of the 
cerebral ganglion, the oesophageal commissure and the 
ventral cord, unsegmented and paired. The sexual organs 
are hermaphroditic. Development with metamorphosis. 
The germs Polygordins differ in a few points. The body 
is not ciliated and without ventral furrow. Oesopha- 
gus simple, with a posterior circle of papillae. Ventral 
cord, a median and unsegmented epithelial thickening. 
Throughout the whole length of the body a contractile 
blood vessel appears, connected by arched and segmental 
cross vessels. Sexes separate. Dinophilus is, perhaps, a 
retrograded Annelide form, resembling Protodrilus. 
Segmental organs without infundibuliun; dissepiments, 
mesenteries and blood vessels wanting; ventral cord only 
little developed. Sexes separate. 

The Chcetopoda are characterized by cuticular bristles 
or setae rarely absent. They are developed in follicles 
(from the external epithelium); each seta being secreted 



CHAETOGNATHA. 141 

by one single cell, which breaks through the follicle and 
projects above the surface of the skin. The follicles are 
deep-seated and provided with special muscles and a 
peritoneal cover. They are of chitinous consistency and 
fibrous structure. The variety of their forms is import- 
ant for classification. We distinguish (a) simple setae, 
consisting of one piece (setae lineares; uncini, curved like 
an S; paleae (glittering): aciculae or needles, within the 
parapodia); by compound setae, derived from the less 
curved uncini, and consisting of two pieces, the stalk and 
movable appendage according to whose manifold form, 
many types are distinguished (lance-shaped, sickle- 
shaped, broom-shaped setae). Compound setae are only 
found among the Errantia. Arrangement of setae is 
definite; in some cases they are added like tufts to special 
appendages of the body (Polychaeta), in others (Oligo- 
chaetha) they are simply inserted in the skin. 

The body of the Chaetopoda is mostly elongated, worm- 
shaped, transversely round or ventrally flattened, The 
segments and their appendages are most important for 
the shape of the body, especially the external appendages 
(1) of the segment; (2) of the prostomium, and (3) of the 
terminal segment. 

The segmental appendages exhibit the most manifold 
modifications. They either consist of simple retractile 
foot stumps containing a bundle of chisel- shaped, hair- 
like setae, and are as such called protopodia (in Sacco- 
cirrus), or the originally simple appendage divides above 
the basal portion into a dorsal and ventral branch, each 
one carrying setae and a cirrus or tactile organ ; to the 
dorsal region gills (branchiae) of simple or complicated 
structure may be attached. Such appendages are called 
parapodia. They are either complete or incomplete 



142 VERMES. 

(certain parts wanting). The basal portion may disap- 
pear when the branches become independent, forming 
bristly knobs and pads, either in two rows or in one. 
When the parapodia and their appendages are com- 
pletely wanting (Oligochaeta) the satae are embedded in 
pits of the integument. Among the various modifica- 
tions of the parapodia in different regions of the body 
the development of peristomial cirri is the most impor- 
tant. The parapodial cirri of the anterior body segment 
are, after the union of this segment with the prosoma, 
developed into tentacular structures. In some cases 
several of the anterior segments undergo such a change. 
The setae are suppressed and the cirri more highly devel- 
oped (Phyllodocidae, Eunicidae, Nereidae. 

The appendages of the prostomium consist of two pri- 
mary tentacles and generally five (typical) cerebral cirri. 
The tentacles are typical in Protodrilus, Polygordius 
and Saccocirrus (supplied with blood vessels, sensory cells 
and very contractile) ; there are similar lasso-tentacles in 
Spiondae (with longitudional furrow). In all others 
they are more or less reduced. The cerebral cirri con- 
sist of a dorsal and ventral pair terminating in a single 
apical cirrus. They are simply a continuation of the 
four longitudinal rows of cirri. But through a displace- 
ment of the prostomium these cirri have assumed a dorsal 
position. They are complete in most of the Errantia ; in 
others retrogression has taken place ; in the Terebellidae 
they are very numerous ; whilst the absence of prosto- 
mial appendages is typical in the Oligochaeta and Echiu- 
ridae. 

The appendages of the periproct are manifold and mor- 
phologically not yet fully explained. They appear in 
the form of anal prongs (in many Archiannelides), in 



CHAETOGNATHA. 1 43 

addition to which a circle of pappillae occur in some 
Polygordiidas, which again lead to anal cirri in Ophel- 
iacea and Spionidae. In the Errantia only a small 
number of large anal cirri is present, representing the 
terminations of the longitudinal rows. In some groups 
only two (rarely one) appear, while others are altogether 
free from them. 

The organization of the Chaetopoda follows closely 
that of the Archiannelides, with rare but important mod- 
ifications. 

The external epithelium secretes a distinct cutiada 
(containing diagonally transverse fibrous structures) 
which is strongly developed in the Errantia. The 
epithelium itself (subcuticula, matrix) consists of sup- 
porting and of glandular cells opening externally by 
pores. Subepithelial glaiids are rare, but may become 
powerful (tubes of Sedentaria). Here also the para- 
podial glands may be mentioned. The ?iervous system 
exhibits all the transitions of lamination, from the per- 
fectly epithelial lamination to complete separation. The 
former is the rule among the Polychaeta, the latter 
belong to the Pligochaeta. The typical parts of the 
central nervous system are : the cerebral ganglion, the 
oesophageal commissure and the ventral trunk. The cere- 
bral ganglion, when epithelial, almost fills the prosto- 
mium (Polychaeta); when separate, it lies above the 
oesophagus (Oligichaeta). Hatschek maintains that it 
is composed of middle brain, tentacular ganglion and 
posterior ganglia. The peripheral nerves proceeding 
from the cerebral ganglion supply the anterior part of 
the body and its sensory organs, which partly join the 
ganglion immediately. The oesophageal commissure lead- 
ing to the ventral trunk consists of nerve fibres and their 



144 VKRM3S. 

enveloped tissues. The anterior parts of the ventral trunks 
sometimes extend to the mouth surrounding it. The ven- 
tral trunk extends through all metameres (except the ter- 
minal) and exhibits a distinct segmentation. Connected 
with it are the transverse commissures and peripheral 
nerves (three in each segment). When the two lateral 
cords of the ventral trunk are far apart and connected by 
long commisures they assume the shape of a rope ladder; 
when they are closely together they constitute the ven- 
tral ganglio?i chain. The epithelial or subepithelial peri- 
pheral nerves often extend to the dorsal line, where they 
unite into a ring. At the base of the parapodia a para- 
podial ganglion is inserted, which, with other peripheral 
ganglion cells, may form a nerve plexus. An oesophageal 
nervous system with ganglia and connected with the cere- 
bral ganglion and oesophageal commissures is generally 
present. A sympathetic nervous ganglion structure for the 
midgut has only been investigated in the Hirudinei. 
Sensory organs are only developed in the Brrantia. The 
integumental sensory organs are generally sensory buds 
scattered over the whole body (special case of Capitel- 
lidae). Sensory buds in the oesophagus may be organs 
of taste. The paired ciliated pits (or pouches) connected 
with the posterior cerebral lobe are considered to be or- 
gans of smell (in Polychseta). Auditory vesicles are rare, 
mostly in the region of the oesophageal commissure. 
Simple vesicular eyes occur generally on the prostomium 
of the Polychseta, connected with the middle brain. 
Their number varies. In some Serpulidae eyes occur on 
the numerous threads of the tentacular crown. Only 
few Oligachaeta (Nais) possess eyes (rudimentary). The 
somatic muscular structure consists of the external circu- 
lar layer, the internal longitudinal layer (a dorsal and 



VKRM^S. 145 

ventral pair with peritoneal epithelium and sometimes 
a medioventral region), the transverse septal muscles 
(Polychaeta) and the muscles of the setae bundles, extend- 
ing in a different direction to the body wall. Visceral 
muscles are not only in the alimentary canal but also in 
other internal organs; the muscles of the dissepimenta 
may also be mentioned here. The directions of the ali- 
mautary canal, foregut, midgut, and hindgut exhibit 
manifold differentiations. The foregut or oesophagus al- 
ways extends through a number of the first anterior seg- 
ments in which no gonads are developed. It frequently 
contains a protrusible pharynx, which in the Archannel- 
ides appears as an anterior ventral appendage, whilst in 
the other groups the oesophagus itself is protrusible. 
At its posterior end there may occur either soft papillae 
(Nephthys) or chitinous teeth, so-called jaws (Glycera). 
The pharynx may also lie in form of a tubular structure 
in the oesophagus (Phyllodocidae) provided with a chi- 
tinous'tooth (Syllidae) or with two lateral jaws (Nereidae). 
In the Bunicidae the pharynx reaches its highest devel- 
opment, consisting of an anterior ventral appendage of 
the oesophagus which is provided with a large number 
of upper jaws and one pair of small lower jaws. The 
Oligochaeta have a short oesophagus without pharynx 
and armature. The midgut consists of the peritoneal 
layer, external, longitudinal, and internal circular layer, 
vascular layer and internal ciliated layer. Segmental dila- 
tations give rise to large blind sacs (Aphroditeae). The 
secondary ventral canal (Capitellidae and Brrantia) and 
typhlosolis (earth worm) as well as the differentiations 
of crop and gizzard (Oligochaeta) may also be mentioned 
here. The hindgut lined by the ectodermal epithelium 
is of small dimension. The blood- vascular system of the 

H 



146 VERMES. 

Chsetopoda shows the typical characteristics with more 
or less important modifications. The Capitellidse and 
Glyceridae have no blood vessels. Generally a closed 
system is present which contains a colored liquid mostly 
without blood corpuscles. The dorsal vessel and often 
also the somatic vasular arches are contractile. The 
blood flows generally from the ventral vessel through 
the somatic arches (containing the branchial vessels) to 
the dorsal vessel, which is either simple or paired, or if 
wanting replaced by an alimentary blood sinus. Gill 
hearts occur in the vascular arches of Eunice; in many 
Oligochaeta some of these anterior arches are transformed 
into lateral secondary hearts. The smaller forms of 
Chaetopoda possess only the typical blood vessels, 
whilst new vessels and vascular branches in gills and 
body wall appear in the larger forms. The body cavity is 
divided by the typical median longitudinal mesenteries, 
the segmental septa and the transverse septa in a 
number of more or less separated chambers. But the 
introduction of the oesophagus in the anterior segments 
has given rise to a retrogession of the dissepiments and 
medial mesenteries in this region. The dorsal mesentery 
and transverse septa are altogether wanting in the Oli- 
gochaeta. The peritoneal epithelium with which the 
body cavity is lined shows peculiar differentiation (chlo- 
rogogous cells of Oligochaeta). The fluid of the body 
cavity contains lymphatic, colored (resembling blood cor- 
puscles), and excretory cells. The typical segmental 
organs are the excretory organs, whose infundibulum pro- 
trudes into the next anterior segment. In the Polychaeta 
they are short and wide, mostly straight, occupying a 
retroperitoneal position outside of the body cavity. In 
the Oligochaeta they are always looped and coiled and 



VERMES. 147 

located within the body cavity. Their canal is often dif- 
ferentiated. Of the many modifications we mention the 
occurrence of sexual organs in one segment, each organ 
having several infundibula corresponding to the groups 
of setae (exotic Lumbricidae); in the same animals a net- 
work of ciliated canals is connected with the segmental 
canals. In the Capitellidae the anterior segmental organs 
perform the function of genital ducts in the Terebellidae, 
whose body cavity is divided into an anterior and pos- 
terior cavity by a single specially developed dissepiment; 
the organs of the posterior segments become eductory 
genital passages. Special complications arise in L,anice 
conchilega. 

The genital organs of the Oligochaeta consist of one or 
two (Lumbricidae) pairs of testes and of one pair of 
ovaria, the former always situated before the latter. They 
generally occupy the space between the ninth and four- 
teenth segment. The sexual ducts are separated from 
the germ glands and show great structural resemblance 
to the nephridia with sexual function. The male appa- 
ratus consists of the testes early dissolving into the 
mother cells of the spermatoza; of the sperm sacs (ex- 
cept in Chaetogastridae), into which these mother cells 
are early received and become mature; and of the vasa 
deferentia (different numbers), each of which consists of 
a preseptal funnel and of a sperm duct, which perforates 
the septum and opens externally. In some Lumbriculidae 
a sperm capsule arises between the sperm sacs so that 
they become mere appendages. The preseptal funnels 
lie in the same segments as the sperm sacs or capsule. 
The female apparatus consists of the two ovaria, the two 
oviducts (not present in lower forms) and the receptacula 
seminis (invaginations of the integument). The eggs 



148 VERMES. 

either mature in the ovaria or the ovaria separate into 
groups of egg cells, only one of which develops into an 
egg. In some cases germ sacs intervene between the 
ovaria and the oviducts. The receptacula seminis are 
in no connection with the sexual apparatus. They open 
externally and receive the spermatozoa through the ex- 
ternal opening. 

The sexes of the Polychaetae are with few exceptions 
separate. The ovaria or testes arise during certain periods 
from the entothelium of the body cavity. The position 
of the germ glands changes in a remarkable degree. 
They now arise on the genital plates now on the dissepi- 
ments, now on the mesenteries, now as cumulations of 
the entothelial sheath of the ventral vessel, etc. The 
ovaria or testes repeat themselves in several segments ; 
their form varies as much as their position. The eggs 
or spermatozoa mature swimming free in the body cavity 
and are expelled through genital ducts (nephridia). 

The development of the Polychseta and Archiannelides 
is indirect (free swimming larvse), that of the Oligo- 
chseta direct (without larvae). A number of the former 
groups are viviparous the eggs developing within the 
body cavity or within an uterus which is in reality a 
modified segmental organ. Various arrangement for 
breeding are also developed. Some expel their spawn 
in large gelatinous masses, others deposit the eggs with- 
out any special protection. Segmentation is unequal, 
but may come near the equal type. In the latter case a 
cceloblastula arises with thickened entoderm or an epibo- 
lic gastrula, in fact all transitions between the different 
types of segmentation occur. Kupomatus (Hatschek) 
offers a good illustration of embryonic development. 
The spherical egg divides into eight blastomeres of 



VERMES. 149 

almost equal dimensions. The segmentation of the 
animal pole is much more rapid than that of the 
vegetative, consequently the blastomeres of the latter 
remain so much larger. A blastula results in which 
the cells of the three germ layers are already differ- 
entiated. The upper hemisphere (smaller cells) fur- 
nishes the ectoderm ; the lower largely the entoderm, 
two of its cells (primitive mesoderm cells) early assum- 
ing a spherical form and developing the mesoderm. 
The position of the mesoderm cells corresponds to the 
anal termination of the larva. An equatorial circle of 
cilia (later preoral of the larva) arises, and later the 
ciliated tuft of the larval vertex appears. The ento- 
dermal part of the blastula invaginates. At the same 
time the primitive mesodermal cells move into the in- 
terior and afterwards divide. The two original cells are 
still distinguished from the newly produced cells by their 
larger size. Hatschek calls them the pole cells of the 
mesoderm; they lie at the end of the two new meso- 
dermal streaks. In the further development of the larva 
the alimentary canal curves towards the anal side and 
unites later with the depression of the ectoderm which 
furnishes hindgut and anus. In the meantime the blasto- 
pore had considerably contracted, forming a split with a 
small opening through which the ectoderm entered the 
interior, forming the oesophagus. Then follows, as an 
enlargement, the stomach of the larva, and after this the 
small intestine and hindgut. Already during gastrula- 
tion the embryos had arisen from the bottom to the 
surface of the water, and external changes took place 
together with the internal. The trochophora stage is 
reached. To what has been said before on the trocho- 
phora we might add that here the so-called cephalic kid- 



150 VKRMKS. 

ney, the paired excretory organ, arises from the meso- 
dermal streaks, i. e., from a few cells situated near the 
pole cell, which elongate and become hollow, so that 
the cephalic kidney reaches from near the anus to the 
oesophagus. It consists of a ciliated canal, which may 
divide, and one or more funnel-shaped blind termina- 
tions. The eye spot of the Kupomatus larva is situated 
in a cell of the vertex region, asymmetrically on the 
right side. The two ectodermal vesicles represent sensory 
organs which arise symmetrically at the posterior part of 
the body from each ectodermal cell respectively. They 
have the character of otolith vesicles. A perianal circle 
of cilia is formed in Bupomatus, wanting in many other 
larvae of this group. The larva is transformed into the 
adult worm by a gradual reduction of the anterior region. 
Polygordius may serve here as an illustration. Seg- 
mentation manifests itself, due to a remarkable change 
of the mesodermal streaks, which through cell division, 
have grown quite considerably. Each one divides into 
two cell layers, extending towards the ventral and dorsal 
line. Anterior segmentation takes place, and the two 
layers of the streak separate, a cavity arising in each 
segment. Thus primitive segments originate, whose 
external and internal wall become the somatic and 
splanchnic layer in every segment of the anus, and whose 
adjoining walls form the segmental limits (dissepiments) 
of the worm body. The 'primitive segments belonging 
to each segment unite in the median line of the ventral 
and dorsal surface and form a ventral and dorsal mesen- 
tery. Upon the vertex plate of the head region the two 
very small ciliated tentacles arise. The originally 
vesicular midgut has elongated with the body and become 
cylindrical. The posterior circle of cilia appears imme- 



VERMES. 151 

diately before the anus. Gradually the cephalic region 
decreases. The change of the vertex plate and the con- 
traction of the head wall conditions the change of the 
large cephalic sphere into the slender head of the worm. 
The vertex plate has assumed the shape of a cone ; the 
eyes appear more distinct than in the larva. The pro- 
ductive segments of the trunk have increased in number 
and the segmentation of the body has become more dis- 
tinct. 

In the last stage of development the segmental con- 
strictions of the alimentary canal make the segmentation 
still more definite. The head sphere and the ciliated 
apparatus have disappeared entirely in this stage, and 
the worm is complete in its main features ; it gives up 
the larval habit and adapts itself to the crawling loco- 
motion. 

The larvae of the Annulata are of very various forms 
because segmentation begins at phylogenetically younger 
stages than the trochophora represents. They are distin- 
guished according to the distribution of their cilia as 
atrochse, rnonotrochae, telotrochae, mesotrochae andpoly- 
trochae, nototrochae and gasterotrochae. Amphitrochae 
are those which consist of dorsal and ventral semicircles. 

The development of the Oligichceta is direct. The fer- 
tilized eggs are deposited in solid chitinous cocoons of 
various form and habit. The number of eggs contained 
in them (swimming in an albuminous mass) differs ac- 
cordingly. Only some of them develop at the expense- 
of the others. Segmentation is always unequal ; but 
eggs, poor in nutritive yolk, give rise to an invagination 
gastrula ; where the opposite is the case, an epibolic 
gastrula develops. The rise^ of the mesoderm streak 
is represented by two blastomeres which enter the seg- 



152 V3RMKS. 

mentation cavity generally before the formation of the 
two primary germ layers (but apparently in relation 
to them). The mesoderm streaks themselves arise when 
the two cells divide and the resulting smaller cells 
separate from them. They extend along the two sides 
of the embryo towards the mouth, gradually moving to 
the ventral surface until they occupy both sides of the 
median line. Thus the embryo has considerably in- 
creased in size, and represents essentially a cell sphere of 
two layers between which the mesoderm streak is ven- 
trally inserted. The blastopore has become a definite 
mouth around which a lip-like thickening of the ecto- 
derm arises. Contractile cells around the mouth cause 
through their swallowing motions the filling of the ali- 
mentary canal with albumen and a consequent enlarge- 
ment of the embryo. The egg membrane bursts and the 
embryo swims in the albuminous mass of the cocoon. 
In this stage it resembles the free swimming larva of the 
Annelides especially when an oral ring of delicate cilia, 
or an adoral ciliated zone arises. In some cases even a 
cephalic kidney is present. All these facts seem to in- 
dicate that the endnyos of the Oligoch seta are retrograded 
larval forms. The transition of the larval embryo into 
the adult worm results from the further development of 
the mesoderm streaks, which anteriorly grow around the 
fore-gut and enlarge posteriorly from the ventral side to 
the dorsal, thus separating the ectoderm from the ento- 
derm. Segmentation had begun already at an earlier 
period and with it the separation of somatic and splanch- 
nic layer, finally the primitive segments arise pos- 
teriorly and envelope the entoderm sac dorsally which 
completes the chief external form of the worm. A later 
ectodermal depression at the posterior and gives rise to 
the anus. 



VERMES. 153 

Some remarkable cases of sexual reproduction and of 
alternation occur in the Chaetopoda, e. g., in Cte?iodrilus, 
Protula, JVaz's, Antofytens aud Syllis. 

The Echiurida constitute another suborder of the Chae- 
topoda. The form and internal organization of the larvae 
as well as the origin of the setse justify this conclusion. 
The retrogression of segmentation in the adult animal 
and the setae, as well as the enormous enlargement of 
the cephalic lobe or so-called proboscis gives them the 
character of modified forms. The segmentation of Bonel- 
lia is unequal, producing an epibolic gastrula; that of 
Thalassema is equal, resulting in an invagination gas- 
trula. The entoderm represents in both cases a solid 
mass with a differentiated cell layer and an internal yolk 
mass. The larva of Thalassema and Bchiurus corresponds 
closely to the trochophora form; segmentation, however, 
disappears later, and of the fifteen primitive segments 
only the somatic and splanchnic layer remain. The larva 
of the Bonellia may be traced back to the trochophora, 
but it undergoes an important modification. 

The eggs oi My zo stoma (hermaphrodites; sexual ap- 
paratus resembling that of Plathelminth^J are fertilized 
outside of the parental body after the development of 
the two polar bodies. Their segmentation is unequal 
and leads to an epibolic gastrula. The larva greatly re- 
sembles the trochophora, although the absence of the 
preoral circle of cilia prevents a perfect comparison. 
The further development of the larva is very simple. 
A real body cavity is not present; parenchymal tissues 
traversed by muscle fibres takes its place. 

The body of the Hirudinei is elongated, in most cases 
dorso-ventrally flattened, rarely round. The numerous 
circular furrows do not correspond to the internal seg- 



154 VERMES. 

ments; three, four or five external rings belong gener- 
ally to one internal segment, with the exception of the 
opposite terminal rings, which exactly correspond to 
the internal arrangement. The number of segments 
may be determined by the number of these external 
rings, which carry sensory organs and are always con- 
stant. The integument is naked and without append- 
ages (except in Pontobdella and Branchellion). The 
cutaneous muscular structure is characterized by diagon- 
ally intersecting fibres. The pharyngeal apparatus of 
the Rhynchobdellidse corresponds very minutely to that 
of some Plathelminthes; it is a pharynx plicatus which 
protrudes from the mouth opening but is not evaginated. 
In the Gnathobdellidse the muscular wall of the oesoph- 
agus itself is thickened and protrudes in form of three 
longitudinal folds or septa into the lumen. These septa 
may be developed into jaws covered with fine teeths. 
Well developed muscular fibres radiate from the pharyn- 
geal wall to the body wall. The midgut exhibits later- 
ally paired, segmentallly arranged diverticula, the last 
pair of which is very long, extending posteriorly along 
the sides of the hindgut. Here also modifications occur. 
The body cavity has not yet been defined. The space be- 
tween alimentary canal and body wall is filled with con- 
nective tissue or parenchyma, subject to manifold trans- 
formations. The difficulty which arises in determining 
the body cavity is largely due to the fact that in all 
Hirudinei the blood-vascular system stands in open com- 
munication with the sinuses, which may or may not be 
lined with an endothelium. Attached to the dorsal and 
ventral body wall muscle fibres extend through the par- 
enchyma, which branch at both ends and form meta- 
merically arranged muscle dissepiments between the 



VERMES. 155 

alimentary diverticula. The structure of the nervous sys- 
tem is in a general way the same as that of the Chseto- 
poda. Important specific characteristics are found in the 
sensory organs. The eyes of worms in general need not 
necessarily be homologous, except the larval eyes of the 
trochophora type, which either disappear or are reduced. 
We mention here especially the five pairs of eyes found 
on five sensory rings of Hirudo medicinalis. The 
structure of the Hirudinean eye is characteristic. It is 
cylindrical, situated almost vertically upon the hypo- 
dermis. The nerve enters at the basal end, its fibrillae 
changing into long sensory cells, which lie in the axis 
of the eye. Large, muscular, pellucid cells surround 
this axis, and the whole eye is embedded in a highly 
pigmented connective substance. In the sensor}* organs 
the long sensory cells are of hypodermic character ; the 
large, transparent cells are comparatively few, and the 
pigment is wanting. Their function is problematic. 
On the lips organs of taste occur. The nephridial struc- 
ture of the Hirudinea resembles that of the Oligochaeta, 
except that in the segments of the opposite ends the 
constant nephridia are wanting. The blood-vascular sys- 
tem consists of four longitudinal vessels of a supra- 
intestinal, a ventral (in which the nerve lies), and of two 
(pulsating) lateral vessels. The ventral vessel may be 
looked upon as the chief part of a reduced bod}* cavity. 
A network of lateral branches may connect the longi- 
tudinal vessels at both ends. The peripheral vascular 
system consists of two networks of capillaries, one ex- 
tending through the integument, and the other along 
the alimentary canal. The organs of excretion and 
reproduction are richly supplied with blood vessels. 
The blood consists of colorless amoeboid corpuscles and 



156 VERMES. 

free nuclei. Many modifications occur. In all other 
respects the Hirudinei resemble closely the Chaetopoda, 
They are hermaphrodites. Several paired testes lie seg- 
mentally arranged in the median body region between 
the successive lateral diverticula of the midgut, within 
the muscular septa. They all open into a vas deferens 
lying along the whole region. The two vasa deferentia 
converge anteriorly towards the ventral median line, and 
open in a common aperture (sometimes through a penis). 

The female apparatus lies behind the male opening, 
between the vasa deferentia and consists of two ovaria, 
where oviducts either enter the large muscular vagina (2. 
suborder), or lead directly to the outside (1. suborder.) 

The structure and deposition of the eggs is generally 
the same as in the Oligochaeta. Although different in 
detail, the whole course of their development corresponds 
to that of the Chaetopoda (Oligochaeta). The so-called 
germ-streaks of the Hirudinea and the mesoderm streaks 
of the Chaetopoda do not, indeed, appear to be homolo- 
gous structures, but their relation to the embryonic body 
and their later development show that they both may 
be traced back to similar forms ; the mesoderm streaks 
of the Hirudinei assumed a more complicated structure 
through the addition of ectodermal parts. The same 
may be said of the origin and structure of the organs, 
except that the genital organs resemble those of the 
dendroccelous Turbellaria. On the whole, it must be 
acknowledged that the Hirudinei are more highly devel- 
oped forms. 

The systematic position of Branchiobdella is still a 
matter of dispute. Anatomically it resembles the Oligo- 
chseta, but in its development and parasitic habits it 
approaches more closely the Hirudinei. A proper inves- 



VKRMKS. 157 

tigation of the origin of the nervous system and the 
mesodermal bands will reveal its true nature. So far 
nothing definite can be said about it. 

The structure of the Prosopygii has already been de- 
scribed in its most important features. The different 
orders present great variations. The Sipunculacea ex- 
hibit a regular external segmentation, the rings corre- 
sponding to the muscular bundles of the circular muscu- 
lar layer, and to the lateral commissures of the ventral 
nerve trunk. The presence of longitudinal furrows 
divides the external integument into squares. Papillae 
and wartlike projections appear especially along the pro- 
boscis. The proboscis is the anterior* part of the body 
which can be invaginated into the posterior, the mouth 
being situated at its anterior termination. It includes also 
the foregut. The Spunculacea possess a firm cuticula; 
that of Phoronis is delicate, but a chitinous, separate 
dwelling tube is secreted. The tough, cuticular e?ivehpe 
of the Bryozoa (frequently with lid) becomes calcareous 
in many cases. The mantle of the Brachiopoda secretes 
in a similar way a bivalved, generally calcareous shell. 
The shell can hardly be compared with the bivalved shell 
of the Mollusca. The valves are dorsal and ventral; 
each one is symmetrical and the median plane divides 
each one into two lateral congruent halves; the opening 
is anteriorly and the hinge posteriorly situated. The 
hypodermis of the Bryozoa almost vanishes in the thick 
cuticula. In the Priapulidae and Phoronidea the ele- 
ments of the central nervous system and the hypodermis 
cells are so blended together that a line of demarkation 
can hardly be drawn. The general muscular structure of 
the Prosopygii' differs widely. The naked Sipunculacea 
possess a typical cutaneous muscular envelope, consist- 



158 VERMES. 

ing of an external circular and internal longitudinal 
muscular layer, between which a thin, diagonal, fibrous 
layer is inserted. The retractors of the probocis are 
longitudinal muscles, attached in various numbers either 
to its anterior end pr to the cutaneous muscular envelope 
of the anterior or central region of the body and extend- 
ing freely through the body cavity. The cutaneous mus- 
cular layer of the Phoronidea is typically composed of the 
two layers. There is none in the Bryozoa; at least the 
formation of a rigid cutaneous skeleton (shell, cell, ecto- 
cyst) has reduced it to a set of muscles which permit the 
retraction and protrusion of the soft anterior end and its 
tentacles or a contraction of the body stalk movable 
within a tube. The typical cutaneous muscular envelope 
is likewise absent or at least much reduced in the Brachio- 
poda, due to the formation of a shell. Its remnants are 
a few feebly developed transverse and longitudinal 
fibres beneath the integment of the mantle; further, the 
muscles of the arms (protractors and retractors) and the 
longitudinal muscles of the stalk. A system of strong 
dorso-ventrally situated muscles, attached near the hinge 
to the two valves (adductors and divaricators), serve the 
purpose of opening and closing the valves. They cannot 
be looked upon as modified parts of a cutaneous envelope. 
The alimentary canal is generally well developed 
(blind in the Testicardines). The foregut is very short 
and not strongly differentiated, because the taking in of 
food is mostly accomplished by special external appen- 
dages. Only the Priapulidse exhibit a well developed 
pharynx with numerous teeth and a strong muscular 
structure. In the Phoronidea, Bryozoa and Brachio- 
poda the foregut simply represents the* rarely distinct 
and muscular connection between mouth and stomach, 



VERMES. 159 

commonly called oesophagus. Whilst the midgut of the 
Priapulidae extends as a straight tube through the bod}-, 
that of the Sipunculidae bends at the posterior termina- 
tion and returns to the anterior region, winding spirally 
around the first portion. A ciliated furrow runs along 
the midgut; near the end a blind diverticulum takes its 
place corresponding to the secondary canal of the Chae- 
topoda. Phoronis exhibits a similar descending and 
(dorsally) ascending midgut. In the Bryozoa a stomach 
intervenes which sometimes lengthens into a blind sac. 
The midgut of the Brachiopoda consists of an anterior 
portion of the stomach, into which the oesophagus leads, 
and a gastric canal. lateral evaginations of the stom- 
ach form voluminous lobes, (the liyer) which com- 
pletely envelop it. The gastric canal forms a simple 
or complicated loop variously ending. The two anal 
organs of the Priapulidae, serving first as excretory 
organs then as genital canals open externally near the 
anus. In Sipunculus rudimentary anal glands have been 
observed opening into the last part of the hindgut. The 
anus of the Sipunculidae lies dorsally between proboscis 
and body wall; in Phoronis and Bryozoa without or 
within the base of the tentacle; in the Brachiopoda, 
when present, to the right of the mouth (Crania ex- 
cepted). In the Priapulidae it is situated at the posterior 
end. The body cavity of the Sipunculacea is large and 
spacious; dissepiments are wanting; the alimentary canal 
is fastened to the body wall by delicate mesenterial 
cords (absent in Priapulidae). In Priapulus the body 
cavity continues into the tail appendage. The alimen- 
tary peritoneum of the Sipunculidae is largely ciliated, a 
longitudinal band of muscles extends along the canal. 
The body fluid contains amoeboid lymph cells, genital pro- 



160 VERMES. 

ducts and other unknown substances. In Phoronis a sys- 
tem separates the cavity of the cephalic lobe from the body 
cavity ; mesenteries attach the alimentary canal to the 
body wall. The body cavity of the Bryozoa is reduced 
in some cases and well developed in others. Muscular (?) 
fibres fasten the alimentary canal to the body wall ; one 
or two funiculi attach the blind ccecum to the posterior 
body wall. The body cavity of the Brachiopoda is lined 
by a largely ciliated entothelium. A dorso-ventral mes- 
entery dividing the cavity into two lateral halves fasten 
the alimentary canal to the body wall. Sometimes a 
^astro-parietal band and a ileoparietal band are also 
present in the region of the hindgut. These bands have 
been compared to septa which originally divided the 
body into three segments. The body cavity continues 
into the mantle cavities. The nervous system of the 
Sipunculacea begins with an oesophageal ring around 
the anterior end of the alimentary canal, joining dorsally 
a well developed brain (only thickened in Priapulidse), 
whence commissures are given off to the anterior integu- 
ment and the tentacles. Nerves proceed from the 
oesophageal ring innervating the alimentary canal (two 
in Sipunculus) and the pharynx (4 in Priapulus). The 
ganglionic ventral cord sends off lateral branches which 
dorsally unite into rings, corresponding to the external 
rings ; or simple regular segmental swellings of the 
ventral cord occur (Priapulus). The nervous system of 
the other Prosopygii is very little developed. The ven- 
tral cord is reduced to the lower oesophageal ganglion, 
but even this can be wanting. This peculiarity is pro- 
bably due to their sessile habit which caused a degenera- 
tion of the sensory organs. Phoronis possesses a nerve 
which proceeds from the dorsal side of the oesophageal 



VKRMKS. 161 

ring and extends, lying- asymmetrically on the left side, 
along two thirds of the body ; inside of it a tube (noto- 
chord?) occurs. In the Brachiopoda nerves enter two 
by two the arms, the mantle and the retractory and pro- 
tractor} 7 muscles of the body, w T hich branch out again. 
In the Bryozoa nerves enter the tentacles. The nephri- 
dial system of the Prosopygii is but little developed. 
Never more than two pairs of nephridia occur. They 
serve as ducts for the genital products and may also have 
excretory functions. They open near the arms in the 
Sipunculidse ; they are wanting in the Bryozoa (except 
Entoprocta) ; in the Brachiopoda they are situated to the 
right and the left of the mouth and open in the mantle 
cavity. So-called anal tubes occur in the Sipunculacea, 
either open or closed, serving the same purpose as the 
nephridia. As organs of respiration serve the tentacles 
or the oral arms which are either penetrated by blood 
vessels or by vascular processes of the body cavity. Also 
the integument and tail processes may perform the same 
function. The blood-vascular system is wanting in the 
Priapulidae and the Bryozoa. Sipunculus possesses a 
dorsal and a ventral vessel accompanying the foregut 
and opening anteriorly into a circular sinus embracing 
the mouth cavity at the base of the tentacles ; posteriorly 
it ends in the region where the retractor muscles of the 
proboscis are attached to the body wall. There is pro- 
bably an open communication between body cavity and 
vascular system. In Phoronis a closed vascular system 
with red blood corpuscles is developed, consisting of a 
dorsal and ventral vessel, with anterior circular and 
many posterior, lateral, blind vessels; the gastric canal is 
surrounded by a blood sinus. All vessels are contrac- 
tile. The existence of an apparatus of blood circulation 



1 62 VERMES. 

in the Brachiopoda is disputed. It has however been 
repeatedly confirmed that a contractile tubular heart lies 
above the gastric cavity and a vein proceeding from 
above the foregut. There are also other blood vessels 
claimed to be present. Ova and spermatozoa of the 
Sipunculidae, Phoronidae and Brachiopoda arise at cer- 
tain points from the entothelium of the body cavity. The 
genital products fall into the body fluid and are emptied 
through the nephridia. The anal ducts of the Priapu- 
lidae are first excretory organs but become afterwards 
germ-glands and eductory passages. The sexes of the 
Bryozoa are either represented by different persons, or 
they are present in one. The ovaria of the Ectoprocta 
arise mostly as cell cumulations of the internal body 
wall, the testes on the funiculus. The statoblasts of the 
parthenogenetic Bryozoa are parthenogenetic eggs and 
located on the funiculus. Eggs and spermatozoa fall into 
the body cavity. How they are emitted has not yet been 
determined with any degree of certainty. The first 
stages of the development of Sipunculus are not known. 
Hatschek obtained the pelagic embryos in the blastula 
stage which already shows the structure of the three 
germ layers. The free swimming larva is more highly 
developed than the trochophora of the Polychaeta to which 
it generally corresponds. The characteristic preoral cir- 
cle of cilia is however wanting (present in Phascolosma), 
but the postoral circle is highly developed. The charac- 
teristic anterior position of the anus is due to the ex- 
tremely rapid growth of the postanal region of the body 
as compared with the preanal. There is no cephalic 
nephridium present. The mesoderm is, however, much 
further developed than in the trochophora of the Poly- 
chaeta. The spacious body cavity does not correspond to 



VERMES. 163 

the primary bod} 7 cavity of the trochophora, but to the 
seeondaty body cavity of the Annulata which develops 
within the mesoblast bands. Segmentation does not 
take place. The development of Phascolosoma is much 
simpler. Here an original epibolic gastrula changes into 
a kind of invagination gastrnla. The larva coincides 
more closely with the trochophora; segmentation is like- 
wise wanting. This latter fact necessitates the separation 
of the Sipunculidae from the Echiuridae, with which they 
formed the Gephyrea, a subdivision of the Annelid es. 

It is a very difficult matter to assign ^he Bryozoa their 
proper place in classification. We have already said, 
under the head of Entoprocta, that they have to be sepa- 
rated from the Bryozoa, because the\* exhibit a different 
phylogenetic origin. However, although the larva of the 
Ectoprocta (of Membranipora) is apparently of a very 
different structure it is, nevertheless, reducible to the same 
type as that of the Entoprocta, but the latter retain in the 
structure of their bodies and the formation of their colo- 
nies essentially the organization of the Bryozoan larva. 

The larva of Phoronis is known by the name Artino- 
trocha. Mouth and anus are situated at opposite ends of 
the ciliated larval body. A large cephalic lobe extends 
over and beyond the mouth, the margin of which is sup- 
plied with larger cilia; the whole resembling the preoral, 
ciliated ring of the trochophora. The vertex plate of the 
cephalic lobe is supplied with four eye spots. Behind 
the mouth are two successive rings of larval and adult 
tentacles respectively. At the base of the latter the nerve 
ring of the adult is formed. Back of them, at the ventral 
side, lies the embryonic adult body projected into the lar- 
val body, which fact distinguishes this larva from that of 
Sipun cuius, which it otherwise resembles. On either 



1 64 VERMES. 

side a nephridium is developed, corresponding to the 
cephalic nephridia of the trochophora and forming the 
nephridia of the adult Phoronis. Around the anus a 
thick circle of cilia is situated. The Actinotrocha sinks 
to the bottom, the body trunk is evaginated and the ali- 
mentary canal formed. The cephalic lobe, the vertex and 
the larval tentacles are cast off and eaten by the young 
Phoronis. The remaining part of the larval body grows 
little, and the anus retains its dorsal position near the 
mouth. 

The free swimming larva of Argiope (Brachiopoda) 
consists ol three successive segments, the anterior, mid- 
dle and end segment. The umbrella- shaped anterior 
segment contains four eyes ; its margin has larger cilia 
than the rest of the surface. A dorsal and ventral fold 
attached to the middle segment covers the terminal seg- 
ment. In either side of the free edge of the ventral fold 
two bundles of bristles occur. Only the midgut is devel- 
oped. A paired secondary body cavity, mesenteries and 
muscles, is situated between alimentary canal and exter- 
nal larval integument. The larva fastens itself by the 
end of the terminal segment which grows into a stalk. 
The two folds develop into the mantle, within which the 
reduced anterior segment is contained. The bristles are 
cast off. The stomodaeum arises from a deepening of 
the anterior body wall, whose base breaks through and 
into the anterior end of the midgut. The eyes degenerate. 
No traces of system and nephridia observed. The larva 
of Terebratulina shows a close relationship with that of 
Phoronis. It manifests itself in the formation of a ten- 
tacular disc on the dorsal mantle lobe and its lengthen- 
ing into two processes, the arms which carry the cirri. 
The umbrella- shaped anterior segment probably corre- 



VHRMKS. l6l 

sponds to tlie vertex of Phoronis, and the terminal seg- 
ment of trie Brachiopedal larva to the evaginating stalk 
of Actinotrocha. The oral nerve rings of the two may- 
be homologous. 

The Chcetognatha resemble externally the Rotatoria. 
The cutaneous muscular envelope is very highly devel- 
oped, owing to the extraordinary rapidity with which 
these animals move through the water. The circular 
layer, however, is wanting (Spadella excepted). The 
cephalic muscular arrangement is very complicated. 
Alimentary canal very simple. The nervous system is 
well developed. The central nervous system and the 
peripheral nerves (one exception) lie outside of the mus- 
cular structure within the body epithelium. We dis- 
tinguish a supra- and a larger infra-cesophageal gan- 
glion, connected by two large commissures. From 
the former proceed : two interior motor nerves, two 
lateral integumental nerves, two optic nerves and two 
olfactory nerves, extending to the sensory organs behind 
the eyes. From the infra-cesophageal or ventral gang- 
lion riumerous lateral nerves are given off (to auditory 
vesicle) and the oesophageal commissures extend as two 
strong nerve cords along the ventral region of the body, 
sending out numerous lateral nerves which, together 
with the ganglion branches, form a nerve plexus ex- 
tending beneath the whole body- epithelium. The 
motor nerves develop lateral cephalic ganglia with 
small accessory ganglia, which together enter the muscles 
of the head and the anterior arm. The paired oviducts 
and spermducts in the posterior end of the body may be 
looked upon as transformed nephridial organs. Organs 
of respiration and blood-vascular system are wanting. The 
Chaetognatha are hermaphrodites. The ovaria are sit- 



1 66 V3RMKS. 

uated in the lateral chambers of the anterior body cav- 
ity attached by mesenteries which at the same time 
envelope the oviducts (blind at the anterior end) that 
open externally near the septum between body and tail. 
The male apparatus resembles that of the Chaetopoda, 
Sipunculidse, Phoronis and the Brachiopoda. The two 
testes, situated along the lateral lines of the tail wall, 
empty the immature spermatozoa into the lateral cham- 
bers of the tail cavity, where they mature. They leave 
the body through the vasa deferentia, which consist of a 
ciliated funnel, a duct and a dilatation. The eggs of the 
Chsetognathaare spherical, transparent, and contain nu- 
merous translucent little yolk spheres. Segmentation is 
total and equal, resulting in a regular blastula, which 
develops into an invagination-gastrula. Two large cells, 
the so-called genital cells, appear early at the bottom of 
the ccelom-invagination, opposite the blastopore. The 
plane which separates these two cells corresponds to the 
later planes of symmetry. The characteristic peculiari- 
ties in the development of the Chsetognatha are, chiefly: 
The origin of the mesoderm through the formation of 
the two coelom-diverticula and the early separation of 
the genital structure. Their embryonic and larval stage 
does not resemble the corresponding trochophora stages, 
which important fact distinguishes them from the Annel- 
ides. 



ARTHROPODA. 



Bilaterally symmetric animals with chitinous exoskele- 
ton, segmented body and joined segmented appendages 
on all or on a number of segments ; with brain, oesopha- 
geal commissure and segmented ventral nerve cord ; with 
heart situated above the alimentary canal ; sexes sepa- 
rate ; with a pair of sexual glands and originally paired 
eductory passages. 

I Sub-branch: BRANCHIATA. Aquatic animals. 
With the exception of the anterior antennae all append- 
ages are originally biramous. Respiration through in- 
tegument or gills which are almost always basal attach- 
ments of the body appendages. 

Only Class : Crustacea. 

I Subclass : Entomostraca. The body consists of a 
variable number of segments. Frequently it is divided 
into an anterior portion with and a posterior portion with- 
out appendages. Both, however, may consist of a vari- 
able number of segments. The genital openings generally 
lie along the line which divides the two main body 
regions. A shield-like carapace is frequently present, 
but variously developed. Appendages manifold in form. 
Together with lateral eyes, the single frontal eye is re- 
tained in the adult animal. Chitinous stomach want- 
ing. Development with metamorphosis. A Nauplius 
larva leaves the egg. Mostly small Crustacea. 



1 68 arthropoda. 

i. Order: Phyllopoda. Swimming feet with branchial 
sacs ; mandibles without palpi ; maxillae reduced. 

i. Suborder: Bra?ichiopoda. Body distinctly seg- 
mented ; with numerous segments and (paired) swim- 
ming feet. Carapace either flat or forming a bivalve 
shell, rarely wanting. Heart an elongated dorsal vessel 
with numerous pairs of ostia. I^ive in fresh water. 
Branchipus (without shell). Apus (with flat carapace). 
Estheria. Limnadia, with bivalve shell. * 

2. Suborder: C/adocera (JDaphflidse) . Water- fleas. Body 
small, indistinctly segmented, with few segments and 
four to six pairs of swimming feet. The posterior an- 
tennae are large paddle-like feet. Gill sacs may be want- 
ing. With a bivalve shell. Head freely projecting. 
Female with dorsal brood pouch between shell and 
body wall. Heart sac-shaped with one pair of ostia. 
Mostly in fresh water. Daphnia. Sida. Moina. Lynceus. 
Polyphemus . Leptodora. Evadne (marine). 

2. Order: Astracoda. Body small, consisting of few 
segments, indistinctly segmented, with bivalve shell. 
Besides the five anterior pairs of cephalic appendages, i. 
e., the antennae, mandibles and maxillae, all or some of 
which may serve as crawling or swimming feet, there 
are only two pairs of legs present. Heart present or 
wanting. Fresh water forms: Cypris. Marine forms: 
Cy there. Halocypris. Cypridina. 

3. Order: Copepoda. With biramous or swimming feet. 

1. Suborder: Eucopepoda. Body small, mostly 'dis- 
tinctly segmented, without shell- reduplicature. The 
shell consisting of only ten segments, the five anterior of 
which carry five pairs of biramous swimming feet, whilst 
the anterior ones are without appendages. First anterior 
segment fused with the head. Antennae, mandibles and 



BRANCHIATA. 1 69 

maxillae (posterior maxillae dissolved into its branches) 
well developed, at least in the non-parasitic forms; the 
parasitic forms with sucking or piercing mouth parts. 
Heart wanting; sac-shaped, when present. The females 
carry the fertilized eggs in a paired or unpaired ovarian 
pouch. Gills wanting. Non-parasitic Copepoda or Com- 
mensals: Cyclops. Canthocamptus (fresh water). Ceto- 
chilus. Clatisocalanus (marine). Notodelphys, commensale 
in the branchial cavity of Ascidia. Parasitic Copepoda: 
Corycceus. Sapphirina (some only temporary parasites). 
Chondracanthus . Caligus. Lemcea. Lemceocera. Penella. 
Lernanthropus. Lerncsascus. Achtheres. Anchorella. 

2. Suborder: Branchiura (Argulidce). Carp-lice. The 
body consists of the shield-shaped, flattened cephalo- 
thorax and the short, flattened, longitudinally split ab- 
domen. In front of the suctorial mouth-tube a long, 
protrusible stylet. Four pair of long, cirrus-like, bira- 
mous, swimming feet. Two large compound lateral eyes. 
Testes in the tail fin. Heart present. Females without 
egg pouches glue their eggs to foreign objects. Argulus, 
upon the carp. 

4. Order: Cirripedia. Characteristic of sessile forms. 
Body indistinctly segmented, fastened by the head end, 
enveloped by a calcareous mantle, forming a shell or a 
case. Anterior antennae (organs of attachment) of min- 
ute size, posterior ones reduced. Mouth appendages 
small, partly reduced. Six (more rarely four) pairs of 
long, biramous cirripedes. Without heart. Hermaph- 
rodites, sometimes with dwarf males, more rarely of sep- 
arate sex and dimorphous. Live in the ocean. 

1. Family: Lepadidtz (Pedunculated). Head end drawn 
out into a long attached peduncle. Lepas. Conchoderma. 
Scalpellum. Pellicipes. Ibla. 2. Family: Balanid<z. Pe- 



170 ARTHROPODA. 

duncle wanting. Body surrounded by an external ring 
of plates. Balanus. Tubicinella. Coronula. 3. Family: 
Alcippidcs (Abdominalid). Body surrounded by a flask- 
shaped, soft mantle, with three or four pairs of feet cor- 
responding to the last four pair of the other Cirripedia. 
Live in the calcareous shells of other Cirripedia and of 
the Mollusca. Alcippe. Cryptophialus. 4. Family: 
Proteolepadida (Apoda). Body grub- shaped, without 
cirripedes. Anterior (clasping) antennae band- shaped. 
Mouth suctorial. Alimentary canal rudimentary. Para- 
sites in the mantle of other Cirripedia. Proteolepas . 5. 
Family: Rhizocephala {Kentrogonidd) ought perhaps be 
separated as a special suborder or order from the other 
Cirripedia. Body tubular; only corresponding to the 
head region of related Crustacea Integument split into 
two lamellae, between them a brood-cavity, opening 
through an aperture in the external lamella. Alimen- 
tary canal wanting. Appendages wanting. Hermaph- 
rodites with dwarf males. Parasites in the abdomen of 
Decapoda. The saccular body bears a peduncle, with 
branched root-like filaments, which penetrate the interior 
of the host whence they carry the food to the body of the 
parasite. The larval stages are similar to those of the 
other Cirripedia (Nauplius and cipris-like larva). Sac- 
culina. Peltog aster. 

II Subclass: Malacostraca. The body consists of 
three regions, with a constant number of segments. The 
head, originally composed of five segments ; the thorax, 
with eight segments; the first anterior, or more than 
one, or all of them, may fuse with the head into a 
complete or incomplete cephalo-thorax ; and the abdo- 
men, composed (with the telson) of seven segments (of 
eight in Nebalia). All the segments of the thorax, 



BRANCHIATA. 171 

with the exception of the last (seventh and eighth in 
Nebalia), bear appendages. The extreme anterior tho- 
racic feet frequently approach the mouth opening as 
maxillipeds, and perform as such functions of nutri- 
tion. The sixth pair of pleopods is almost always of 
different structure, and constitutes frequently, together 
with the telson, a tail -fin. The lateral portion of the 
posterior part of the cephalic segments are free and 
developed as overhanging folds. A pair of compound 
lateral eyes and a chitinous stomach (crop) are present 
in all cases. The female sexual apertures are situated 
on the antepenultimate, the male on the last pair of 
the thoracic segments. Development either with or 
without metamorphosis. The larva leaving the egg is 
rarely a Nauplius. 

1. Legion : Leptostraca. A very important group 
of Crustacea which stands nearest (among all living 
forms) to the ancestral type of the Malacostraca, and is 
frequently placed as a special subclass between the Euto- 
mostraca and Malacostraca. Body slender, covered by a 
bivalved carapace, except the four last abdominal seg- 
ments. Besides a movable cephalic plate. Head, with its 
five pairs of typical appendages distinctly separate from 
the thorax. All eight segments of the short thorax dis- 
tinctly marked, with eight pairs of uniform, biramous, 
lamellous, thoracic feet. At the basal portion of the proto- 
podite a large epipodial lamella (serving as a gill). The 
four anterior pairs of pleopods are strong, biramous 
swimming feet, the two posterior pairs are short, bira- 
mous. The last segment of the abdomen bears two forked 
processes. Two stalked, compound, lateral cephalic 
eyes. Heart elongated, with seven pairs of ostia, extends 



172 ARTHROPOD A. 

through thorax and abdomen as far as the fourth abdom- 
inal segment. Chitinous stomach (crop) present. 

Only order and family: Nebalidcs. Nebalia. Paraneba- 
lia. Nebaliopsis. Marine. 

Related to the L,eptostraca, are the fossil, palaeozoic, 
Ceratiocaridce ( Archaeostraca) : Hymenocaris ', Ceratrocaris, 
etc. 

2. Region : Arthrostraca (Edriophthalmata). Cara- 
pace wanting (except in Anisopoda). The first 
thoracic segment (rarely the second) fused with the 
head, and the first anterior pair of thoracic feet corre- 
spondingly changed into maxillipeds. The two lateral 
eyes sessile. 

i. Order: Anisopoda. First and second thoracic seg- 
ment fused with the head. Cephalo-thorax with over- 
hanging carapace, covering on either side a branchial 
cavity. The two pair of maxillae with palpi. The palpi 
of the anterior pair protrude into the branchial cavity. 
On the maxilliped an epipodial appendage serving as a 
gill. The feet of the second thoracic segment (fused 
with head) are developed into strong pincers. Abdomen 
with biramous swimming feet. Heart in thorax, gener- 
ally with two pair of ostia (in Apseudes only three). 
Apseudes. Tanais. Leptochelia. 

2. Order: Isopoda. Body broad, often dorso-ventrally 
flattened. Only the first anterior segment fused with 
the head, the other seven free. No separate carapace 
The two pair of maxillae without palpi. Abdo- 
men generally short, often reduced, in most cases 
consisting of six segments with two branching lamellous 
pleopods, whose endopodites (generally) serve as gills. 
Heart in the abdomen, often extending into the posterior 
thoracic region, with one or two pairs of ostia. Cymo- 



BRANCHIATA. 1 73 

thoida, hermaphrodites, either parasitic (fish), or non- 
parasitic: Cymothoa. Anilocra. Cirolana. Nerocila. 
Aega, almost exclusively marine forms. Sph&romidce 
free, mostly marine : Sph<zro?7ia. Pranizidce, free in the 
ocean, the first three thoracic segmeuts fused with the 
head. Anceida, females parasitic, males free : Anceus. 
Idotheidcz, free, largely marine: Idothea. Asellidtz : 
Asellus, fresh water form ; Oniscidce, terrestrial. Oniscus. 
Porcellio. The groups of the Bopyridcs and Cryptonis- 
cidcB contain parasites, which are largely hermaphrodites, 
with dwarf males. Females deformed. Bopynts (with 
separate sexes). Gyge. Entoniscus. Cryptoniscus . etc. 
3. Order : Amphipoda (flea- crabs). Body laterally 
compressed. In the typical Amphipoda only the first 
anterior thoracic segment, in the Caprellidse and Cya- 
midae the first two are fused with the head. The gills 
are tubular epipodial appendages of the thoracic feet. A 
well-developed abdomen bears six pair of biramous feet ; 
the three anterior (stronger) ones serve as swimming 
feet, the posterior (posteriorly directed) are adapted for 
springing. Heart in thorax, generally with three, rarely 
with one or two pair of ostia. 

1. Suborder: Crevettina. Head and eyes small. The 
maxillipeds fuse to form a large underlip with two pedi- 
form palpi. Marine forms: Corophium. Talitrus. Or- 
chestia. Lysianassa. Fresh water form: Gammarus. 

2. Suborder: Hyperina. Head and eyes large, the 
latter often divided into frontal and lateral eyes. Maxil- 
lipeds from a small underlip without palpus. Marine 
forms: Hyperia (eyes not divided). Phronima. Platys- 
celus Oxycephalus. 

3. Suborder: L&modipoda. Abdomen rudimentary 
and apodal. The two anterior thoracic segments fused 



174 ARTHROPODA. 

with the head. Gills on the second and third free 
thoracic segments, legs of these segments often reduced. 
Marine forms. CaprellidcB. Body very thin and slender 
Caprella. Proto. Protella. Cyamidce. Body broad and 
flat. Parasites on the skin of whales. Cyamus. 

3. Region: Thoracostraca (Podophthalmatd) . With a 
carapace which covers a larger or smaller portion of 
the thorax and fuses with the dorsal integument 
of all or some of the anterior thoracic segments, but al 
ways remains along the sides as a gill cover of the 
respiratory cavity. A variable number of anterior thor- 
acic segments, or all of them fused (at least dorsally) 
with the head into an incomplete or complete cephalo- 
thorax. The two lateral eyes (except in Cumacea) 
stalked. 

1. Order: Cumacea. Shell (cephalo-thoracic shield) 
small, leaves the five posterior free thoracic segments 
uncovered. Eyes sessile, closely approaching or fused 
into one, but little developed, sometimes wanting. Two 
pairs of maxillipeds. The first one with very large epi- 
podite which bears a gill. Of the six following pairs of 
thoracic feet the two first ones always contain, besides 
the entopodites, also an exopodite; this is often the case 
with the three next pairs, but never with the last one. 
Abdomen long and slender. In the females the pleo- 
pods, except those of the last pair, are wanting. Marine 
forms. Diastylis. 

2. Order: Stomatopoda. Cephalo- thorax rather small, 
not covering the three posterior distinct thoracic seg- 
ments. Body elongated, dorso-ventrally flattened. 
Abdomen large and well developed. The five anterior 
pairs of thoracic feet, situated near the mouth (mandibles 
and maxillipeds) are organs of prey, with epipodial lamel- 



BRANCHIATA. 1 75 

lae, but without exopodites. The three posterior pairs are 
birainous, without epipodial appendages. The five pairs 
of anterior pleopods are strong lamellous swimming feet, 
whose exopodites bear gill tufts. The sixth pair of ple- 
ods forms, with the telson, a powerful tail-fin. The 
heart lengthens into a dorsal vessel, transversing the 
abdomen, and provided with several pair sof ostia, ovaria 
and testes in the abdomen. Marine. Squilla. 

3. Order: Schizopoda. Cephalo thorax well developed, 
thin, covering the whole thorax; the dorsal integument 
of the five last or of the last thoracic segment is not fused 
with it. The eighth pair of thoracic feet are almost uni- 
form and represent biramous feet (with exopodite and 
entopodite), but the two anterior pairs of abdominal feet 
may be called maxillipeds or foot-jaws, because they 
play the part of jaws. Abdomen strong, slender. 
Pleopods of the female very small, in the male very 
strongly developed. The last pair, well developed in 
both sexes, forms with the telson, a rowing or swimming 
fin. Marine. 1. Family: MysidecB. Thoracic feet with- 
out gills, the first pair with large swinging epipodial 
lamella. The five last thoracic segments free beneath 
the dorsal shield. Auditory organs in the entopodites 
of the sixth pair of pleopods. My sis, Siriella: (male 
with gills on the pleopods). 2. Family: Lophogastridcz . 
With gill tufts on the thoracic feet ; five last thoracic 
segments as in Mysideae. Lophogaster. 3. Family: 
Euphausidce. With gill tufts on the thoracic feet. Only 
the last abdominal segment is free under the dorsal 
shield. Euphausia, Thysanopoda. 

4. Order: Decapoda. Cephalo-thoracic shield large, 
mostly firm and hard; calcareous, covering the whole 
thorax and fused with the dorsal integument of all 



176 ARTHROPODA. 

thoracic segments. Kxopodite of the second maxilla 
forms a vibrating plate which regulates the water cur- 
rent in the branchial cavity. The three anterior pair of 
thoracic feet are developed into maxillipeds, the five 
posterior (some armed with pincers) adapted for walking 
(therefore Decapoda). The ambulatory feet of adults only 
consist of protopodite and entopodite; the exopodite is 
wanting; auditory organs at the basal joint of the interior 
antennae. Development direct or with metamorphosis. 
In the latter case the larvae leaving the egg are generally 
advanced beyond the Nauplus stage passing through the 
Zoea and Mysis stage. Extraordinarily rich in forms. 

1. Suborder: Macrura. With well developed abdomen, 
which is at least as long as the cephalo-thorax. Mostly 
with the full number of pleopods, the last pair of which 
forms with the telson a strong tail-fin. Candida: Penceus. 
Palcemon. Crangon. Po?itonia. Alpheus. Sergestes. Leu- 
cifer. Marine. Astacidcs: Astacus fluviatalis (cray fish 
in rivers). Homarus (lobster). Nephrops. Callianassa. 
Gebia. PalpinuridcB : Palinurus. Scyllarus. Marine. 

2. Suborder: Anomura (not sharply distinct from the 
preceding). Abdomen tolerably large, tail-fin mostly re- 
duced. The last or the two last pair of ambulatory feet 
rudimentary. Third maxilliped long and pediform. Pa- 
gyridcs : Hermit crabs, marine, living in snailshells. Ab- 
domen with soft integument, assymmetric with rudimen- 
tary pleopods serving as chelae. Pagurus. Eupagnriis. 
Birgus (in holes in the ground). Hippidce, marine, living 
in the mud. Abdomen with solid integument, generally 
bent forward. The Galateidce {Galatea) resemble the 
Macrura, the Porcellanidtz {Porcellana) the Brachyura. 

3. Suborder: Brachyura. Body short and widened. Ab- 
domen without anal fin, reduced, bent upon the ventral 



ARTHROPODA. 1 77 

side of the cephalo thorax. In the male only the two an- 
terior pairs of pleopods. Notopoda, marine. Dromia. 
Dorippe. Lithodes. Oxystomata (round crabs): Maja. 
Pisa. Stenorynchus. Inachus. Lambrus. Marine: Cyclo- 
metopa (bow crabs). Telphusa (in fresh water). Cancer 
(pocket crabs). Xantho. Pilumnus. Eriphia. Portunus. 
Carcinns. Marine. Catometopa (quadrangular crabs): 
Pinnoteres. Ocypoda. Grapsus (marine). Gecarci?ius (land 
crabs) . 

i. Appendix. I Trilobita. No living species in ex- 
istence. They all belong to the palaeozoic age. The 
integument of the dorsal side is hard ; the ventral side 
soft. The body is divided into three regions : cephalic 
shield, thorax and tail- shield (pygidium). Two longi- 
tudinal, almost parallel furrows, divide these three parts 
into a convex middle region (rhachis) and two lateral 
regions (pleura). The semi-circular or semi-lunar cepha- 
lic shield bears two large compound eyes. The thorax 
consists of a large number of movable segments, and the 
pygidium of a variable number of more or less fused 
segments. The animal was capable of bending the tail 
back upon the thorax. The appendages are rarely pre- 
served. They are segmented, slender feet which are of 
uniform size from the cephalic shield to the pygidium. 
Four pair of maxillipeds, the anterior one (the strongest) 
inserted back of the (shell-like) upper lip. The other 
appendages are biramous feet consisting of a long ento- 
podite, a short exopodite and a branchial thread- or 
band-shaped epipodial attachment. These gills are rudi- 
mentary in the pygidium. Marine animals. Agnostus 
(two thoracic segments). Trinudeus. Olenus. Para- 
doxides. Conocephalites. Sao. Calymene. Asaphus. 

I* 



178 ART HROPODA . 

Bronteus. Phacops. Cheirurus. Acidaspis. Lichas. 
Prcefus. Harpes. 

II Gigantostraca. (Merostomecs. Eurypteridce) ; en- 
tirely extinguished. They likewise belong to the palaeo- 
zoic age. Perhaps largest of all the Arthropoda. Body 
consists of head (cephalo- thorax?), thorax and abdomen. 
The unsegmented head is relatively small, carrying two 
compound lateral eyes and near the median line two 
ocelli. Thorax and abdomen consist each of six seg- 
ments. To the sixth abdominal segment a styliform or 
fin-shaped telson is attached. Head region with six pairs 
of spiny appendages (not biramous). The first anterior 
pair constitutes either delicate, small antennae (Eurypte- 
rus) or a long chela with powerful pincers ; the basal 
joints of the others are inserted around the mouth. A 
jaw is attached to each of them, extending inwardly. 
The last pair of maxillipeds is much stronger than the 
others and oar-shaped, they evidently served as a swim- 
ming foot. Five plates consisting of two lateral halves 
and arranged like tiles on a roof, cover the leaf shaped 
gills on the ventral side of the thorax. The anterior 
(largest) plate is called operculum. The abdomen is 
without appendages. There is a large oval plate behind 
the mouth, the metasoma. Eurypterns. Pterygotus. 

III Hemiaspidae. Extinct palaezoic forms ; perhaps 
the connecting link between the Gigantostraca and 
the Xiphosura. Head with shield, thorax with five or 
six free segments (rarely fused), abdomen with three or 
more segments, terminated by a strong styliform telson. 
Two compound eyes, no ocelli. Two dorsal longitudinal 
furrows give the thorax the appearance of a Trilobite. 
Appendages unknown. Bunodes. Hemiaspis. Belinarus. 

IV Xiphosura (Pcedlopoda. Limididce). Body covered 



ARTHROPODA. 1 79 

with a thick, chitinous integument. Two parts: cephalo- 
thorax and abdomen, with a large, movable postanal 
styliform telson. Cephalo thorax almost semilunar, very 
large, with two lateral horns, directed posteriorly. On 
its dorsal side two compound eyes and two ocelli in front 
of them. The flat segmented abdomen, attached to the 
cephalothorax by a movable joint, has a somewhat hexa- 
gonal shape. Dorsal side convex, ventral side concave. 
Cephalothorax with seven pair of appendages, the first 
one (small, with pincers) corresponding to a second 
pair of antennae (chelicerae). The five following are 
stronger and longer, with jaw-like processes at the basal 
joint (modified in fifth) and pincers at the end (modi- 
fied in fifth). They arise at the sides of the cleft-like 
mouth opening. Behind the mouth two style- shaped 
processes, the chilaria. The seventh pair of thoracic ap- 
pendages resembles those of the abdomen; they consist 
of two plates (joined in the median line), which cover 
the five pair of leaf-shaped abdominal feet, and are called 
the operculum. Each abdominal foot consists of exopo- 
dite, entopodite and gill (integumental reduplicature). 
The gills are arranged like the leaves of a book. Ab- 
dominal feet also used for swimming. A sternal ento- 
skeleton also occurs. Only genus: Limulus. Marine. 
L . moluccanus : East Indies. L. polyphemus. East Coast 
of North America. 

II Appendix: Pantopoda (Pycnogonidse). Body 
much reduced, consists of rostrum, thorax and abdomen. 
Rostrum divided into a median upper and two lateral 
lower portions. At the anterior end the mouth with its 
three lips is situated. The thorax or main body-trunk 
consists of three fused and three free segments, exhibiting 
lateral processes to which the appendages are joined. 



l8o ARTHROPODA. 

Abdomen unsegmented, short, rudimentary, without 
appendages. The chelicerae terminate in chelae (in 
young animals) or are very much reduced if not wanting 
(in adults); the second pair shorter than the rest or 
wanting; the third pair, egg bearer in the males; often 
wanting in the females. The fourth, fifth, sixth and 
seventh pair are never wanting; they consist of nine seg- 
ments, terminate in claws and are extremely long, which 
gives these animals a spider-like appearance. No bira- 
mous appendages. Marine animals. Nymphon. Palene. 
Phoxichilidium. Ammothea. Pycnogonum. Collossendeis 
gigas, a gigantic form in very deep water. The longest 
appendages may become 30 cm. long, the whole body 
only 8 cm. 

II Sub-branch: TRACHEATA. Terrestrial. Ap- 
pendages not biramous, consisting of one rcw of seg- 
ments. Respiration by tracheae (tubular or fan shaped). 

I Class. Protracheata (Onychophora). Body worm- 
shaped. A pair of preoral antennae on the cephalic 
termination. In the mouth cavity a pair of horny jaws, 
at the sides of the mouth oral papillae (salivary papillae). 
Numerous short, almost rudimentary ambulatory legs. 
Apodal abdomen. Breathing by tubular tracheae, the 
external openings of which are scattered over the whole 
body. With numerous, segmentally arranged pairs of 
nephridia. With coxal glands (legs). Heart an elon- 
gated dorsal vessel with numerous pairs of ostia. 

Only genus: Peripatus. Terrestrial animals living in 
dark damp places, beneath the bark of old trees, under 
stones, etc. P. Cape?isis. On the woody hills of the 
Table Mountains; Cape of Good Hope. P. Edwardsii. 
Venezuela. P. Novcb Zealandice. 

II Class: Antennata. (Myriopoda et Hexapoda.) A 



TRACHKATA. l8l 

pair of preoral antennae, three pair of oral appendages. 
Body either honionomously segmented with numerous 
segmented ambulatory legs, or heteronomously seg- 
mented, the appendages being confined to the three 
segments of the thorax, whilst the abdomen is apodal. 
Head and thorax distinct. Breathing by tubular tracheae, 
whose external openings (stigmata) are segmentally 
arranged. Heart in all homonomous Myriopoda an 
elongated dorsal vessel (with many segmentally arranged 
pair of ostia), traversing the whole length of the body; 
in the Hexapoda, confined to the abdomen. 

i. Subclass: My?iopoda. (Thousand legs.) Homono- 
mously segmented Antennata with numerous body seg- 
ments, which are all, with the exception of the last one, 
provided with feet. Without compound eyes. With 
numerous ocelli. 

i. Order: Symphyla. With not more than twelve 
body segments bearing ambulatory legs. A pair of 
branched tracheae, with the stigmata in the head. Un- 
paired sexual aperture on the fourth segment. Scolo- 
pendrella. Scutigerella (Ryder). 

2. Order: Chilopoda. Body dorso-ventrally more or 
less flattened. Every ring with only one pair of append- 
ages corresponding to a segment. The two maxillae 
separate. The first pair of ambulatory feet moved to 
the head as maxillipeds with poison gland opening from 
the terminal claw. Unpaired sexual aperture on the 
penultimate segment. Family : Scutigeridce. With 
compound eyes. Body consists of fifteen segments with 
legs. Only ocelli. Lithobius. Henicops. Family: Sco- 
lopendridce. With twenty- one or twenty- three segments 
with appendages, (not counting the segment of the max- 
illiped). Body elongated. Scolope?idra. Cryptops. Fam- 



1 82 ARTHROPODA. 

ily: Geophilidcz. Body very long, with 31-173 segments 
with appendages. Geophilus. Himantarium. 

3. Order: Diplopoda (Chilognatha) . Body mostly 
arched. From the fifth segment to the end every ring 
bears two paired appendages, thus corresponding to a 
double segment. The two paired maxillae fused into the 
so-called Gnathochilarium. Without maxillipeds. Paired 
sexual aperture between the second and third pair of 
legs. The legs of the seventh ring in the male modified 
into organs of copulation. Family : Polyxenid<z. Fif- 
teen paired feet, Gnathochilarium rudimentary. Copu- 
latory appendages wanting. Polyxenus. Family: Glo- 
meridce 11-14 e}^es, 19-20 rings, 29-31 pair of feet. 
Polydesmus. Brackydesmzis. Family : Chordeumidce . 30 
rings, 45-50 pair of feet. Atractosoma. Craspedosoma. 
Chordeuma. Family: Lysiopetalidce. Number of rings 
large, indefinite. Lysiopetalum. Family : Julidce. 30-70 
and more rings. Julus. Family: Polyzonidcs. Gnath- 
ochilarium rudimentary. Number of rings not constant, 
30-100 and more. Polyzo?iium. 

4. Order: Pauropoda. Antennae forked with three fla- 
gella. Only one pair of feebly developed maxillae. Ten 
body segments. Nine pair of appendages. Trachaea not 
yet discovered. Sexual apertures on the base of the 
second pair of legs. Pauropus. Enrypauropus (Ryder). 

1. Subclass: Hexopada. Insecta. Heteronomously 
segmented antennae, with an almost constant number of 
segments. The body consists of head, thorax (three 
segments) and abdomen (ten segments). Each of the 
three thoracic segments with one pair of feet. Abdomen 
apodal. Compound eyes almost always present; besides 
ocelli. Opening of the sexual organs always at the end 
of the abdomen. 



TRACHEATA. 1 83 

i. Legion: Apterygota. Without wings. Rudiments 
of abdominal appendages at least in the Thysanura. 
Without metamorphosis. 

1. Order: Thysanura. With ten abdominal segments 
and two or three segmented bristle -like appendages 
(cerci) on the anal segment. Compound eyes and ocelli 
maybe present or wanting. Machilis. Lepisma. Nicoletia. 
Ca mpodea . Iapyx . 

2. Order: Collembola. Spring tails. With six or less 
abdominal segments. Almost always a forked springing 
apparatus at the end of the abdomen. Without com- 
pound eyes. Sometimes with ocelli. Smynthurus. Podura. 
Isotoma . Macro ^toma . 

2. Legion: Pterygota. The second and third thoracic 
segment each with a pair of wings. Forms without wings 
occur, which are to be derived from winged ancestors. 

1. Order: Dermaptera (Fo7'ficulid&) . Ear-wigs. In- 
sects with gradual metamorphosis and biting mouth- 
parts. Last abdominal segment with unsegmented ap- 
pendages (cerci), forming pincers. Anterior wings short, 
modified into horny wing covers. Posterior wings large, 
delicate, fan-shaped; can be folded longitudinally and 
transversely. Sexual openings separate or rudimentary 
on one side. Foyjicula. Labidura. 

2. Order: Orthoptera (straight winged). Insects with 
gradual metamorphosis, with biting mouth- parts, with 
two pair of thin or parchment-like wings (sometimes 
wanting). Anterior wings mostly shorter and more firmly 
chitinous than the posterior wings. Cerci of various 
forms on the abdominal cavity. Sexual aperture is un- 
paired. EmbidcE, Blattid<z, cockroaches: Periplaneta. 
Blatta. Mantidce: Mantis. Phasmidcz: Bacillus. Plasma. 
Pkyllium. Saltatoria with the families of Acridiidcz {Ac- 



184 ARTHROPODA. 

ridium, GLdipoda, Mecostethus, Stenobothrus ■, Melanoplus, 
Tettix, etc.; of the Lociistidce: Locusta. Centhophilus . 
Xiphidium. Orchelimum. Conocephalus Cyrtophyllus . Am- 
blycorphcea. Microcentrum. Scudderia. Thyreonotus, and of 
the Gryllidcz: Tridactylus. Gryllotalpa. Gryllus. Nemo- 
bius. CEcanthus. 

3. Order: Ephemeridea. Insects with incomplete meta- 
morphosis. Mouth parts rudimentary, after the type of 
biting insects. Posterior wings small or wanting, an- 
terior wings large, wing texture thin and membranous. 
Abdomen with three (rarely two) long cerci. Paired 
genital ducts with separate apertures. Larvae apneustic, 
resembling the Thysanura, with tracheal gills and bit- 
ing mouth parts, live in water. Eyhemera. Paling enia. 
Chloe. 

4. Order: Odonata {Libellididce) . Insects with incom- 
plete metamorphosis, with biting mouth parts, Abdo- 
men with two unsegmented anal stylets, Wings large, 
richly netted-veined, glassy. Thoracic legs moved for- 
ward. Larvae in water, with variable tracheal gills, 
apneustic. Libellida. sEschna. Calopteryx. Agrion, 
etc. 

5. Order: Plecoptera {Perlarice) . Insects with incom- 
plete or gradual metamorphosis; with biting mouth parts. 
Abdomen mostly with two long cerci. Both wings large, 
spacious cells between the veins, the posterior pair often 
broader than the anterior and capable of being partly 
folded. Larvae thysonura-like, with tracheal gills, ap- 
neustic, aquatic. The tracheal gills often persist in the 
imago. Per/a. Nemura. 

6 Order: Corrodentia. Insects without any or with 
gradual metamorphosis, with biting mouth parts. Wings 
often wanting. Membranous in the Termitidae, caducous 



TRACHKATA. 1 85 

in their sexual individuals; wanting in the workers. 
Several Psocidse and Mallophaga are wingless. The 
latter also without compound eyes. The winged Psoci- 
dse have glassy wings, with broad-celled veins similar to 
those of the Hymenoptera. Young Corrodentia resem- 
ble the Thysanura. Termitidce, white ants, living in 
communities: Termes. Colotermes. Psocidcz, wood lice. 
Troctes (book louse). Psocus. Mallophaga, bird lice. 
Parasites. Trichodectes . Philopterus. Liotheum. 

7. Order: Thysa7ioptera sive Physopoda. Insects with 
gradual metamorphosis, larva resembling closely imago. 
Last larval stage' takes no food. Sucking mouth parts. 
The claws of the short feet changed with the hooked 
lobe of the tarsus into an evaginable bladderlike appa- 
ratus. Wings very narrow, with reduced system of veins, 
with long fringed margin ; often wanting or rudimentary. 
Only three or four pairs of stigmata (one or two on thorax, 
one on the first and one on the eighth abdominal ring.) 
Nervous system concentrated. Thrips. 

7. Order: Rhynchota. Insects with gradual metamor- 
phosis (in the males of Coccidse complete). Mouth parts 
form a rostrum adapted for piercing and sucking. Com- 
pound eyes wanting in parasitic Rhynchota. 

1. Suborder: Phytophthires (plant lice). With two 
pairs of membranous wings, wanting in the females. 
The Coccidse have only anterior wings, the posterior 
wings being changed into halters. Family : Psyllidoe, 
leaf-fleas, two pairs of wings, the anterior ones parchment- 
like: Psylla. Livia. Family: Aphidce, plant lice, two 
pairs of membranous wings, mostly wanting in the 
female: Aphis. Chermes. • Schizoneura. Phylloxera, 
Lachnus. Myzus. Family: Coccidce, shield lice: Coccus 
Leca?iium. Aspidiotus. 



1 86 ARTHROPOD A. 

2. Suborder: Pediculid& (Aptera). Lice. Without 
facetted eyes and wings. Pediculus. Hcsmatopinus. 
Phthirius. 

3. Suborder: Heteroptera (Hemipter<i) . Bugs. Wings 
(partly wanting). Anterior wing covers horny, tips 
membranous. Posterior wings membranous. Geocores, 
land bugs: Hydrometra. Halobates. Pentatoma. Coreus. 
Corizus. Alydus. Pyrrhocoris. Lygczus. Miris. Capsus 
Acanthia (bed. bugs), Reduyiz, etc. Hydrocores, water 
bugs: Nepa. Ranatra. Naucoris. Corixa. Notonecta, 
etc. 

4. Suborder: Homopiera. Anterior and posterior wings 
uniform, membranous, the front pair generally coriace- 
ous. Cicada. Pseudophana. Centrotus. Aphrophora. Tet- 
tigbna. Ledra, etc. 

9. Order: Neuroptera. Insects with complete meta- 
morphosis and biting mouth-parts. Two pairs of mem- 
branous wings, the nervures of which form a network, 
transparent. Family: Megaloptera. Myrmeleon. Man- 
tispa. Hemerobius. Chrysopa. Family: Sialidce, Larvae 
aquatic with tracheal gills. Stall's. Corydalis. Raphidia. 

10. Order: Pa?wrpata. Insects with complete meta- 
morphosis and biting mouth-parts. Two pairs of mem- 
branous, narrow wings with spacious cells between the 
veins. Larvae caterpillar like. Panorpa. Bittacus. Borens 
(wings rudimentary). 

11. Order: Trichoptera {Phryganidce) . Spring flies. 
Insects with complete metamorphosis. Mandibles rudi- 
mentary. Maxillae form a membranous, obtuse rostrum. 
Body mostly hairy, rarely scaly. Posterior wings gen- 
erally larger than the anterior, can be folded like a fan. 
The grub-like larvae live in dwelling tubes or cases, 
mostly in water, possessing tracheal gills and are apneu- 



TRACHKATA. 1 87 

stic. Phryga?iea. Limnophilus . Halesits. Hydropsyche. 
Mysfaczdes, etc. 

12. Order: Siphonatera sive Aphaniptera. Fleas. In- 
sects with complete metamorphosis, with sucking and 
piercing mouth-parts. No wings. No facetted eyes. 
Parasites. Pulex. Sarcopsylla. Ceratopsyllus. 

13. Order: Coleoptera. Beetles. Insects with complete 
metamorphosis and biting mouth-parts. Anterior wings 
(wing covers) horny Posterior wings membranous, may 
be folded transversely and longitudinally, used exclu- 
sively for flying. Larvae of different form, often resem- 
bling a Thysaneura, sometimes a grub, rarely without 
feet (Curculionidse), with biting mouth- parts. Several 
thousand genera with more than 80,000 species. 

1. Suborder: Cryptotetramera. Tarsi composed of four 
joints, of which one is rudimentary. Families: Coccinel- 
UdcB. Eudomychidics. 

2. Suborder: Cryptopentamera . Tarsi five-jointed, one 
rudimentary and concealed. Families: ChrysomelidcE. 
CerambycidcB. CurculionidcB . Bostrychidce, etc. 

3. Suborder: Heteromera. The tarsi of the two an- 
terior pairs of legs five-jointed, those of the others four- 
jointed. Family: Meloidcs (Cantharidae). Rhipiphoridce, 

TenebrionidcE . CEdemej-idcs . Cistelidce, etc. 

4. Suborder: Pe?itamera. All tarsi generally five- 
jointed. Families: Xylophaga. Mala cod ermata. Ela- 
teridcB. Buprestidce. Lamellicornia. Silphidce. Psela- 
phidcs. Staphyli?iidcB . Hydrophilidcs. Dytiscidce. Cara- 
bidcB. Cicindelidce, etc. 

14. Order: Lepidoptera. Butterflies. Insects with com- 
plete metamorphosis and suctorial mouth parts, which 
mostly form a spirally rolled proboscis. Body covered 
with scales. Both pairs of wings uniform, membra- 



1 88 ARTHROPOD A. 

nous, covered with scales, rarely capable of folding. 
Posterior wings generally somewhat smaller than the 
anterior. L,arval caterpillars with anal feet, with biting 
mouth parts, rarely (Micropteryx) apodal. 

i. Suborder: Microlepidoptera. Families: PterophoridcB. 
Tineidce. Pyralidcs. Tortricidce. 

2. Suborder: Geometrina. L,oopers. Families: Phy- 
tometridce. Dendrometridce . 

3. Suborder: Noctuina. Owls. Families: OphhtsidcE. 
Plusiadce. Agrotidcz. Cuculliadce. Acronyctidce, etc. 

4. Suborder: Bomfiycina (Silkworm moth). Families: 
Bombycidce. Saturnidce. Psychidce. Zyg&nidcz. Cossidce. 
Siparidcs. Eiiprepiadcs . Notodontidcz . 

5. Suborder: Sphingina. (Sphinx moths.) Family: 
Sesiadce. Sphingidcz. 

6. Suborder: Rhopalocera. Butterflies. Families: Hes- 
peridce-. Lyccsnidce. Satyridce. Nymphalidcz. Heli- 
con iidcE . Equitidce . 

15. Order: Hymenoptera. Insects with complete meta- 
morphosis, with biting mandibles, and mostly suctorial 
maxillae. Generally four membranous transparent, 
sparely veined wings. Caterpillars variable, apodal 
(with the exception of the leaf and wood wasps which 
have caterpillar-like larvae), i. e., grubs. 

1. Suborder: Terebrantia. Female with ovipositor as 
tube or borer (terebra). Families: Tentlwedinidcz (leaf- 
wasps). UroceridcB (wood wasps). Cynipidce (gall flies). 
The larvae of: PteromalidcE \ Braccmidce, Ichneumonidce , 
Evaniadce are mostly parasites in the larvae of other in- 
sects. 

2. Suborder: Aculeata. Female with poison sting 
and poison gland. Family: Forrnicidtz (ants). Fossotia 
(digging wasps). Vespidce (wasps). Apidce (bees). 



TR ACHE ATA. 1 89 

16. Order: Diptera. Two-winged. Insects with com- 
plete metamorphosis, with suctorial and frequently also 
stinging mouth parts. Anterior wings membranous, 
transparent. Posterior wings transformed into halters 
(swinging plates). Larvae grub-like (apodal), with or 
without head. 

1. Suborder: Pupipara. Sheep ticks, bird and bee 
lice. Vivaparous. The larvae are born shortly before 
their entering upon the pupa stage. Parasites. Wings 
frequently rudimentary. Melophagus. Braula. Nycter- 
ibia. 

2. Suborder; Brachycera. Flies. Antennae short, con- 
sisting mostly of three segments. Numerous families: 
Muscidce. Co?iopid<z. CEstridcs. Syrphidce. Empidce. Asi- 
lidce. Bomby Hides. Thei'evidcz. Tabanidce. Leptidcz. Xylo- 
phagicUe. Siratiomyidcz . 

3. Suborder: Nemocera {Tipularid). Mosquitos. An- 
tenna long, several segments; males often with tufts of 
hairs. Families: Bibionid<z. Fimgicolcs. Noduiformes. 
C2tlicif0rm.es. CulicidcB. Gallicolce. LimjiobiidcB . 

Note. — The Dermaptera are generally classified as a 
family with the Orthoptera. The Kphemeridea, Odonata, 
Plecoptera Corrodentia and Thysanoptera are frequently 
united under the head Pseudoneuroptera and the Panor- 
pata are united with Neuroptera. 

Ill Class : Arachnoidea sive Chelicerota. Spider- 
like Arthropoda. No preoral appendages resembling the 
antennae of the Antennata and Protracheata. Several 
anterior body segments (seven, including cephalic lobe) 
are fused into an unsegmented region, called cephalo- 
thorax, with six pairs of appendages, of which the first 
anterior one is moved in front of the mouth. The first 
two anterior pairs are developed as oral appendages, the 



190 . ARTHROPODA. 

first one being called chelicerae, the second one pedipalpi. 
The three other pairs of appendages are segmented, 
mostly long legs. Abdomen segmented or unsegmented 
or fused with the thorax, without developed appendages. 
Breath either exclusively by plicate tracheae or by 
plicate and tubular tracheae at the same time, or ex- 
clusively by tubular tracheae. Number of segments 
limited, four pairs at most. The stigmata are almost 
always situated on the abdomen. Heart confined to the 
abdomen, rarely wanting. 

i. Order: Scorpionidea. Scorpions. Body divided 
into a stout unsegmented cephalo thorax and a long, 
segmented abdomen. The latter consists again of a thick 
and broad preabdomen with seven segments and a nar- 
row, elongated postabdomen with five segments. The 
last segment carries a poison sting. On the ventral side 
of the second abdominal segment two lateral comb shaped 
appendages. Chelicerae and pedipalpi terminate in 
claws. Pedipalpi with large chelae, four pairs of pli- 
cated tracheae (lungs) whose stigmata are situated along 
the ventral side from the third to the sixth abdominal 
segments. Euscorpius. Buthus. Androctonus . 

2. Order : Solpugidea. Head separate. Thorax with 
three segments ; abdomen with ten segments, cylindri- 
cal. Chelicerae with claws, pedipalpi long, serving as 
ambulatory legs. Tubular tracheae. A pair of stigmata 
on the first thoracic segment and one on the second and 
third abdominal segment respectively. Galeodes. Sol- 
bug a. 

3. Order : Pseudoscorpionidea (Chernetidce) . Cephalo- 
thorax unsegmented or with two transverse furrows, 
abdomen broad, flat, with eleven segments. Neither 
poison-sting nor caudal spine present. Chelicerae and 



TRACHEATA. 191 

pedipalpi as in second order. Tubular tracheae. Two 
pairs of stigmata on the second and third abdominal seg- 
ment. With spinning glands. Small animals Chernes. 
Chelifer. Obisium. Chthonius. 

4. Order: Pedipalpi {Thelyphonida). Scorpions and 
spiders. Cephalo-thorax unsegmented, distinct from the 
abdomen. The latter pressed down, consisting of eleven 
or twelve segments. Chelicerae claw-shaped. Pedipalpi 
large, terminating either in a claw or in chelae. First 
pair of legs with flagellate termination antenna-like. 
Two pairs of plicate tracheae whose stigmata lie on the 
ventral side of the second or third abdominal rings. 
Thelyphonus (three last abdominal segments form a rudi- 
mentary region distinctly separate from the rest of the 
abdomen, and carrying a long, segmented caudal spine). 
Phtynus. 

Here belong perhaps also the comparatively unknown 
groups of the Tartarid<z and Microthelyphonidce \ 

5. Order: Phalangidea. Cephalo-thorax unsegmented, 
abdomen thick and stumpy, its whole breadth joining 
the cephalo-thorax, consisting of six segments, with 
chelate chelicerae ; the pedipalpi serving as ambulatory 
legs, legs often extraordinarily long and thin. Tubular 
tracheae, with one pair of stigmata which lies ventrically 
along the separation line of cephalo-thorax and abdomen. 
Without spinning glands. Phalangium. Leiobunum. 
Gonyleptus . 

6. Order: Cyphophthalmidea (often classified as a family 
to the preceding order). Cephalo-thorax unsegmented, 
abdomen with eight segments. Habit of the Pseudo- 
scorpionidae. Chelicerae and pedipalpi similar to those 
of Phalangidae. Tubular tracheae. Cyphophthalmus 
(without spinning glands, with one pair of stigmata on 



192 ARTHROPODA. 

the ventral side of the first abdominal segment). 
Gibbocellum (spinning glands on the base of the abdomen 
behind the sexual aperture ; two pairs of stigmata on the 
second and third abdominal segment ) 

7. Order: Araneidea. Spiders. Cephalo-thorax and 
abdomen unsegmented, the latter large and oval shaped. 
Abdomen separated b}^ a narrow short stalk from the 
cephalo-thorax. Four to six pairs of spinning mam- 
millae at the end of the abdomen. Chelicerae chelate 
with poison gland. Pedipalpi serving as ambulatory 
legs. Terminal segment of the male modified for pur- 
poses of reproduction. Either exclusively plicate tracheae 
or both plicate and tubular tracheae. 

1 . Suborder : 1 ' etrapneumones . With four plicate 
tracheae, without tubular tracheae. The two pairs of 
stigmata ventrally behind the base of the abdomen. 
Mostly four (Atypus six) spinning mammillae. Mygale 
{A vicularia) . Cten iza . A typus. 

2. Suborder: Dipneumones. With two plicate tracheae 
which open with two stigmata on the base of the abdo- 
men and with tubular tracheae, opening in an unpaired 
(rarely paired, Dysderidae) stagma, behind the stigmata 
of the plicate tracheae. The unpaired stigmata of the 
tubular tracheae has moved far back, now lying before 
the spinning mamillae. With six spinning mammillae. 
Here belong most spiders spinning webs. Family : Dys- 
deridcB (two stigmata for the plicate tracheae). Dysdera. 
Segestma. Family: Saltigiadtz, jumping spiders: Sal- 
ticus. Attics. Family : Citigradcs (Lycosidae), wolf 
spiders: Lycosa. Tarantula. Family: Laterigradce ', 
crab spiders : Micrommata. Philodromus. Xysticus. 
Family: Tubitelarice •, tube weavers: Dictyna. Tegenaria. 
Agelena. Argyroneta. Dtassus. Cltibiona. Family : 



traCheata. 193 

Retitelaiia:, net spiders: Linyphia. Theriditim. Phol- 
cus. Family: Orbitela? i<z } orb weavers: Epeira. Zilla. 
Meta. 

8. Order: Acarina, Mites. Abdomen fused with the 
cephalo-thorax. Body unsegmented. Biting or stinging 
and suctorial mouth parts. Respiratory organs wanting 
or, if present, tracheae. Many mites are parasites. A. 
Mites with tracheae. Family: Trombidiida ; Trombid- 
iutn. Family: Tetrayiy chides: Tetranychus. Family: 
HydtaclmidcE, water mites : A tax. Hydtachna. Hydro- 
doma. (The subfamily of Halacaridae, sea mites [Aletes. 
Halacarus] without tracheae). Family: Bdellidce. Bdella. 
Family: Oribatidce: Oi'ibata. Leisoma. Family: Ga- 
masid<z, beetle-mites: Gamasus. Uropoda. Family: 
Ixodidcz, ticks : Ixodes. Argus. B. Mites without 
tracheae. Family : Tyro%lyphid<z, cheese-mites : Tyro- 
glyphus. Family: Derntaleichidcz: Listrophorus , Anal- 
ges. Family: Sarcoptida?, itch-mites : Sarcoptes. Fam- 
ily: DemodicidcB: Demodex (in hair follicles of domestic 
animals and man). Family: Phytoptidcz, gall-mites. 
Phytoptus. 

Appendix : Linguatulida (Pentastomidd). Body worm- 
like, mostly flattened, externally segmented. No mouth 
apparatus. Two pairs of movable hooks around the 
mouth. Sensory organs and tracheae are wanting. Male 
aperture behind the mouth, female aperture on the pos- 
terior end of the body. Parasites. Pentastomum. P. 
tcznioides. In the nose and mouth cavity and cerebral 
sinus of dog and wolf. Embryos are expelled with the 
mucous of the nose and swallowed by hare or rabbit or 
other mammals, through whose alimentary canal they 
penetrate the liver and lungs. Here it encysts and un- 
dergoes a remarkable metamorphosis, resulting in the 
J 



194 ARTHROPODA. 

larva Pentastoma denticulatum. It breaks through the 
capsule 'and moves freely about, finally entering again 
the mouth of a mammal. 

Organization of the Crustacea. The external structure 
of the Crustacea is of the highest interest for comparative 
anatomy. The chitinous envelope which covers the 
whole body and its appendages, does not only serve as a 
protection for the internal organs, but also as a skeleton 
to which the muscles are internally attached. There 
are, therefore, intimate relations between external and 
internal organization. We consider: (i) the main body- 
region, (2) the appendages, (3) the gills. 

The m.iin body-region, i. e., the body, without its ap- 
pendages consists of a number of successive segments (met- 
ameres), which are connected by movable joints, the chit- 
inous envelope remaining thin and soft between succes- 
sive rings. Originally the number of body segments 
corresponded to the number of appendages, the an- 
terior or head segment being distinguished by the 
presence of eyes, mouth, brain and antennae (different 
from the other uniform appendages), whilst the last seg- 
ment (without appendages) contained the anus. But no 
known Crustacean exhibits such a regular segmentation. 
On the contrary, the anterior region of all Crustacea is 
externally unsegmented, and bears not one, but five 
pairs of appendages. One cephalic segment may have 
fused with four successive segments. This unsegmented 
anterior region is called the head. All the rest of the 
body may be termed the trunk. (See classification for 
distinction of subclasses with regard to number of seg- 
ments.) We may assume that the same segments corre- 
spond in different Crustacea, e. g., the second or sixth 
or tenth trunk segment of an Isopod to the second, sixth 



Crustacea. 195 

or tenth of a Phyllopod. Likewise, since the segments 
differentiate ontogenetically from the anterior* to the pos- 
terior end, as in the Annulata, it may be assumed t that 
the anal segment is the same in all Crustacea. The regu- 
larity of segmentation may be either disturbed or disap- 
pear altogether: (1) through the appearance of a shell 
or shield (integumental reduplicature), which is very 
general and may be an ancestral peculiar^ of the Crus- 
tacea (segmentation disappears by fusion); or (2) trunk 
appendages may acquire nutritive functions, and join 
the cephalic appendages so that their segments fuse with 
the head segment ; or (3) other appendages may lose 
their functions (locornotory) and degenerate, which 
results in a reduction of the corresponding segments ; or 
(4) acquired parasitic modes of life may cause a degen- 
eration of appendages, and with it the disappearance of 
corresponding segments. 

In discussing the body appendages it will be necessary 
to contrast those of the first anterior segment with all 
others. They consist of a single row of successive joints, 
whilst all the others divide into two branches, called 
biramous feet. Such a biramous foot is composed of 
three parts: the piotopodite (basal), the entopodite (inner 
branch), and exopodite (outer branch). The protopodite 
consists of a proximal part, articulating with the body, 
and a distal part bearing the two branches, which are 
also segmented. Various modifications of the appendages 
arise all of which, however, may be traced back to the 
above type. 

The body of the youngest Crustacean larva, the Nau- 
plius larva, is unsegmented, possessing three constant 
pairs of appendages (1, pair simple feet; 2 and 3, biram- 
ous). The anterior pair develops in the adult into the 



I96 ART\HROPODA. 

anterior antennae, the second pair into the posterior an- 
tennae, the third pair into the mandibles. During the 
metamorphosis (shedding of skin) of the Nauplius the 
larval body elongates and develops posteriorly new seg- 
ments with new biramous appendages. In the adult the 
structure of these biramous feet may change. 

The appendages of the head consist of the anterior an- 
tennae (antennulae), posterior antennae, mandibles, an- 
terior maxillae and posterior maxillae. The antennulae 
are situated before the mouth, and consist only of one 
single row of joints (organs of touch, or smell, or hear- 
ing). The antennae (feelers) consist typically of proto- 
podite, entopodite and exopodite. The mandibles (or 
masticatory apparatus) are originally typical biramous 
feet; the basal joint of its protopodite is modified into a 
hard, often toothed plate; all other parts are reduced. 
The anterior maxillae (masticatory) have retained more of 
the original character, an exopodite being frequently 
present. In the posterior maxillae (masticatory) the ex- 
opodite is still further modified into a swinging plate. 
In the Thoracostraca, the Ostracoda and the Copepoda a 
pair of peculiar processes (paragnatha) arise between 
mandibles and maxillae, which may be the free proximal 
joints of the anterior maxillae. 

The appendages of the trunk are divided into those of 
the thorax and those of the abdomen. Anterior thoracic 
feet may become auxiliary organs of nutrition (maxilli- 
peds). In a complete thoracic foot of a Malacostracan 
an epipodite joins the proximal part of the protopodite, 
whilst the exopodite and the pentamerous entopodite 
are attached to the distal part. When exopodite and 
entopodite are wanting the thoracic foot is simple, repre- 
senting a leg of seven joints in one row. When the 



CRUSTACEA. 1 97 

proximal part fuses with the thoracic skeleton, only the 
distal part of the protopodite can be recognized. The 
abdominal feet (pleopods) are joined to the first six seg- 
ments of the abdomen, whilst the seventh or last seg- 
ment, the telson, is without appendages. 

Respiratio?i takes place through the external integu- 
ments, which in the higher forms undergo various modi- 
fications. The surface of the integuments may be enlarged, 
forming a mantle or a dorsal shield, a bivalve shell or a 
cephalo- thoracic shield, whose soft parts aid respiration 
either as epipodial attachments of the maxillae or of the 
maxillipeds, or as so-called branchial attachments of the 
fourth, fifth and sixth pair of appendages. In the higher 
Malacostraca theintegumental reduplicature serves purely 
as a protection of the delicate gills. In the Balanidse 
numerous mantle processes are generally considered as 
gills. In the Cyprididae a series of gill-leaves occurs 
beneath the shell on either side of the dorsal line. The 
function of respiration is in the large majority of Crus- 
tacea carried on by the appendages or branches of the 
appendages, because the motions of the appendages and 
the consequent change of water is beneficial to respira- 
tion. (See classification for details.) 

The body-epithelium is represented by the hypodermis 
which secretes a chitinous cuticula, serving as a protec- 
tion for the body, and as an exoskeleton for the muscular 
structure. It undergoes in different individuals, in dif- 
ferent places, manifold modifications, in some of them 
becoming calcified. Frequent molts further the growth 
of the body, and represent the metamorphosis of the 
Crustacea. Fine pores perforate the cuticula. 

A connected cutaneous muscular envelope, character- 
istic in the Vermes, is wanting in all the Arthropoda. 



198 ARTHROPODA. 

The exoskeleton offers a much larger localization of the 
muscular structure. There were evidently in the homo- 
nomously segmented ancestors of the Crustacea four 
longitudinal segmented muscles present, two dorsal and 
two ventral, with intersegmental myomeres. The mus- 
cular structure of living Crustacea may be divided into 
that of the body, of the appendages, and that which is 
common to both. The principles involved in the ar- 
rangement and work of the muscular structure of all the 
Arthropoda may be understood if we observe; (1) that 
the exoskeleton represents a chitinous tube in every seg- 
ment of the body and in every joint of the appendages; 
(2) that the muscles are attached to the internal wall of 
the skeleton; (3) that the muscles extend between the 
successive joints; (4) that the chitinous cuticular skel- 
eton is thin and flexible between two segments or joints, 
and (5) that the tubiform exoskeleton of two successive 
segments or joints is connected hinge-like at opposite 
places. 

When the muscular attachments to the exoskeleton 
are of a chitinous nature, an entoskeleton may arise, 
A shell muscle is present in the bivalve Entomostraca for 
the purpose of closing the shell. The muscular cells of 
all Arthropoda are cross-striped. 

The alimentary canal extends as a simple straight tube 
through the whole length of the body. The mouth is 
situated on the ventral side of the head, distinguished 
by an overhanging upper lip and lower lip (paragnatha), 
and surrounded by appendages which have assumed 
the functions of mastication and prehension (mandibles, 
maxillae, maxillipeds). The anus is situated in the ter- 
minal segment of the body. According to the ontogeny 
and structure, three separate parts of the alimentary 



CRUSTACEA. 1 99 

canal must be distinguished: foregut, hindgut, and mid- 
gut, the last forming the long connection between the 
first and the second. Foregut and hindgut, arising from 
the ectodermal stomodaeum and proctodeum of larva or 
embryo, possess on the internal surface of their epithelial 
wall a chitinous cuticula (intima) which continues at 
the mouth and anus into the hypodermal chitinous exo- 
skeleton of the body. Only the epithelium of the mid- 
gut, derived from the mesenteron, is of entodermal 
origin. The midgut develops in almost all Crustacea 
diverticula which play the part of a hepato-pancreas. 
The foregut of the Bntomostraca represents a simple 
oesophagus; that of the Malacostraca an ascending nar- 
row oesophagus and an enlarged, cephalic masticatory 
stomach or crop. Salivary glands are generally absent 
in the oesophagus (except in Astacus fluviatilis), but 
present in the buccal cavity. The midgut of the Deca- 
poda, Isopoda and probably the Anisopoda is almost en- 
tirely transformed into the hepato-pancreatic glandular 
tubes, so that the hindgut becomes very prominent in 
size. The anus of the Bntomostraca is dorsally situated, 
that of the Malacostraca ventrally. Posterior glands or 
diverticula are wanting. Circular muscles control the 
contraction and expansion of almost the whole aliment- 
ary canal. 

The 7iervous system of the Crustacea must be derived 
from that of the Annulata. Typically it consists of the 
brain or supra- oesophageal ganglion (two symmetriclateral 
halves in the first segment of the body — head-segment), 
which sends nerves to the unpaired eye, the anterior 
antennae and the frontal sensory organs. All the other 
segments possess each two ganglia in the median ventral 
line (closely together or double ganglion). The two. 



200 ARTHROPODA. 

ganglia of each segment are connected with each other 
by a transverse commissure, and with the corresponding 
ganglia of the neighboring segments by a longitudinal 
commissure. The two first commissures constitute the 
oesophageal commissures. From each double ganglion 
of the ventral cord nerves are given off to the muscles of 
the segment and its appendages, beginning with the sec- 
ond pair of antennae. Modifications of this general 
structure (best represented in Phyllopoda) arise: (i) 
through a gradual fusion of the double ganglion into 
one; (2) through the close approach of the longitudinal 
commissure, which connect the ganglia; (3) through the 
close approach of successive ganglia which may fuse into 
a ganglion mass (Copepoda); (4) through the displace- 
ment of ganglia; (5) through the displacement of those 
regions which give off nerves; (6) entire disappearance 
of ganglia only takes place in the posterior ganglia. 
All these modifications are due to transformations in the 
whole organization. A sympathetic nervous system oc- 
curs in certain members of the Leptostraca, Arthrostraca 
and Thoracostraca. Nerves branch out from a ganglion 
on the upper side of the stomach, extending to the stom- 
ach wall, the liver, and the heart. 

The eyes of the Crustacea are generally well developed, 
and exhibit a high degree of complication. Organs of 
vision are either wanting or rudimentary in most para- 
sitic and sessile groups (Cirripedia), so among those 
which live in deep or dark regions. We distinguish two 
kinds of eyes : the unpaired frontal eye (secondary) and 
the paired lateral eyes (primary). The former occurs in 
the young larvae of all Crustacea (Nauplius eye), and is 
retained in the adult Bntomostraca ; it evidently belongs 
to ancestral types. Paired eyes occur in all Malacos- 



CRUSTACEA. 201 

traca and in many Entomostraca. They are either 
movable stalked eyes or rigid and sessile ; the former 
are an outgrowth of the latter. They do not constitute 
a pair of appendages. (See structure of the eye, p. 57.) 
Specific organs of touch are the bristles of the antennae 
and other appendages ; one or two ganglion cells at their 
base are connected with the nervous system. Organs of 
smell are represented by special vesicles, threads, tubes 
or styles, grouped in tufts along the anterior antennae. 
They are directly connected with nerve fibres (Calceoli of 
Amphipoda?) Frontal sensory organs are cuticular 
appendages innervated by the frontal nerves. They are 
present in the larvae, and therefore of ancestral type. 
Auditory organs occur in the Decapoda at the basal joints 
of the antennulae (page 53). 

The blood- vascular system is not closed; it stands in 
open communication with blood-lacunae, which are 
open cavities between the different organs of the body, 
representing the body cavity The typical plan of the 
system of circulation (nearest in Branchippus) is as 
follows : A contractile, tubular dorsal vessel (heart) 
traverses the body along the dorsal median line of the 
alimentary canal ; flow of blood, as in Annulata, from 
the posterior to the anterior end. In each trunk- 
segment a pair of slit-like openings, so-called ostia, lead 
from the dorsal vessel in the surrounding blood sinus, 
the pericardial sinus, which again represents a part of the 
body cavity. The haeniolymph flows from the latter 
into the dorsal vessel; leaving this at its anterior end, it 
traverses in reverse order the system of lacunae, which 
extends beneath the integument of the bod) 7 and its 
appendages, where respiration takes place, and returns 
again into the pericardial sinus. All other heart forms 

j* 



202 ARTHROPODA. 

have arisen through reduction of segments and dis- 
appearance of ostia. (Details given in the classification.) 
The organs of respiration are in all Arthropoda interposed 
in that part of the blood circulation which returns the 
blood from the body to the heart. The blood corpuscles 
are generally colorless and of amoeboid form. 

The organs of excretion are chiefly represented by the 
shell gland and the antennular gland. Both are paired, 
the latter opening at the basal joints of the posterior 
antennae, the former situated in the shell reduplicature 
or in the cephalo-thoracic shield, in the region originally 
corresponding to the second maxillary segment ; it opens 
at or near the posterior maxillae The antennular 
glands are present in the adult Malacostraca and the 
larval Kntocostraca ; the reverse is the case with the 
shell glands. The structure of both glands is essentially 
the same. A terminal vesicle (with lateral evaginations 
in higher forms) and loop- shaped urinary canal (with 
complicated coils and urinary vesicle in higher forms) 
compose these organs. Their origin is still doubtful. 

The Crustacea are, with few exceptions, animals of 
separate sex. Only sessile and parasitic forms (Cirripe- 
dia and Isopoda) are hermaphrodites. Parthenogenesis 
occurs only in Estheria and Apus (Brachiopoda) and in 
the Cladocerse. Male and female organs consist of one 
pair each, are of the same typical structure and occupy 
the same position in the body. The male organs are 
composed of the testes, vasa deferentia, ductus ejacula- 
torius and the apparatus of copulation ; the female organs 
of ovaria, oviducts, receptaculum seminis (vulva, vagina), 
and an apparatus of copulation. Ovaria and testes can 
not be distinguished in their earliest stages ; the}' arise 
as cell groups from the mesoderm, so likewise, but sepa- 



CRUSTACEA. 203 

rate from them, the oviducts and vasa deferentia. The 
terminal portions are evaginations of the external integu- 
ment. The external organs of copulation are either 
transformed appendages, or attachments of them, or 
processes, folds, humps, etc., of the integument. 

Modifications of the sexual organs consist largely of 
fusions of the different parts. The ovaria and testes are 
either simple or branched or coiled tubes or sacs, situ- 
ated dorsally on both sides of the alimentary canal, or 
between this and the heart, sometimes occupying the 
whole length of the body. The sexual apertures are on 
the ventral side (except in Copepoda and some Clado- 
cerae). The genital segment of the Entomostraca is not 
constant ; it generally lies immediately behind the an- 
terior part of the trunk. The sexual apertures of the 
Malacostraca have a definite and constant position. 
Those of the male are situated at the basal joints (mostly) 
of the eighth pair of thoracic appendages ; the female 
apertures at the basal joint of the protopodite of the ante- 
penultimate pair of thoracic feet. The spermatozoa are 
often large, of radial shape, and immobile. Frequently 
spermatophores are present, and in the eggs accessory 
envelopes, both secreted by the glands of the eductory 
passages. Oviducts and vasa deferentia, together with 
the antennular and shell glands, are homologous with 
the nephridia of the Annulata. 

Ontogeny of the Crustacea. The study of the meta- 
morphosis of the Crustacea is one of the most interesting 
themes of morphological investigation. Having, how- 
ever, given a full account of the ontogeny of the Annu- 
lata because of their central position, we have to confine 
ourselves to the most essential features. For a long 
time the Nauplius (larval form) was considered by the 



204 ARTHROPODA. 

most prominent investigators the hypothetical ancestral 
form of all Crustacea and the Zoea (larval) occupied the 
same position with regard to the higher Crustacea or 
Malacostraca. To Claus belongs the credit of having 
proved that the Zoea is only a secondarily derived larval 
form, whilst Hatschek first pointed out that in the deri- 
vation of the Crustacea from Phyllopodal ancestors the 
relation of the latter to the Annulata would be the most 
natural explanation. He based his arguments especi- 
ally upon the similar structure of the central nervous 
system, which represents a true homology. A derivation 
of Crustacea from an unsegmented Annelide through the 
Nauplius form, would argue for an independent origin of 
the corresponding features in both groups, i. e., a mere 
analogy, which can hardly be maintained. It is now ac- 
knowledged that the Nauplius is a segmented larval form 
(with three trunk-metameres, a cephalic region and 
an anal segment united with the budding zone) which 
may be compared with a young metamerical segmented 
Annelide larva. This shows that the larvae of the 
Crustacea are trochophorse secondarily modified by the 
early development of typical Crustacean characters. The 
transition of the Annulata to the ancestral forms of the 
Crustacea (Protostraca) was connected with certain 
changes in structure and mode of locomotion. But even 
this hypothetical form differs considerably from the 
Primitive Phyllopoda, the original, real Crustacea. The 
Palseostraca (Trilobita, Gigantostraca, Xiphosura) and 
the Pantopoda constitute the direct descendants of the 
Protostraca. Even Peripatus shows Crustacean charac- 
ters, which proves that the ancestral form of Myriopoda 
and Insecta have their origin in the group of the Proto- 
straca. The primitive Phyllopoda forming the transition 



PERIPATUS. 205 

from the Protostraca to the real Crustacea possessed evi- 
dently a more homonomously segmented body, and less 
difference in the various body regions than we find in 
modern Crustacea. Each trunk segment possessed a pair 
of ventral ganglia, a pair of biramous, lamellate, phyllo- 
podlike appendages, and perhaps also a pair of nephridia 
(as in Peripatus). The forked processes of the anal seg- 
ment may be likewise of ancestral origin. The most 
typical Crustacean characters evidently found their ex- 
pression in the formation of the anterior body region, the 
so-called head, arising from a union of the first five an- 
terior segments (sixth with eyes and frontal organs?), 
whose dorsal integument enlarged into a carapace. The 
originally uniserial antennulae occupy an exceptional 
position as bearers of important sensory organs. The 
second antennae were biramous and served as oars, or, 
perhaps, as masticatory organs. The basal joint of the 
mandibles assumed masticatory functions, whilst the rest 
is still retained in Copepoda as a biramous palpus. The 
two following pairs of maxillae approached the structure 
of the trunk segments which they still possess in 
modern Crustacea. Paragnatha may also have been 
present. The first anterior cephalic segment contained 
the frontal organs (primary cephalic tentacle of Annu- 
lata?) the unpaired eye (so-called Nauplius eye) and the 
paired compound eyes. These are the characters which 
separate the real Crustacea from the Palaeostraca and the 
other Arthropoda. The primitive Phyllopoda were proba- 
bly of separate sex; they possessed a long dorsal vessel 
with paired segmental ostia and hepato-pancreatic diver- 
ticula. 

In addition to what has been said on Peripatus, it 
must be emphasized that its muscular structure is repre- 



206 ARTHROPODA. 

sented by a highly developed cutaneous muscular envel- 
ope, which consists: (i) of an external, circular, fibrous 
layer; (2) of a double layer of intersecting diagonal 
fibres; (3) of a strong, longitudinal, fibrous layer, com- 
posed of various lamina. Numerous sagittal or trans- 
verse muscular fibres constitute a median structure 
(around heart, alimentary canal and genital organs) and 
two lateral structures (around nervous system and seg- 
mental organs). They correspond to the dorso- ventral 
or transverse muscle fibres of the Annulata. Only the 
muscles of the jaws are cross striped. The alimentary 
canal (ciliated internally) extends through the whole 
body, and consists of mouth- cavity (with opening of 
salivary glands), pharynx (with very thick muscular 
wall), short oesophagus, stomach (with folded wall but 
without mesenteries) and rectum. The nervous system 
is distinguished by the numerous transverse commissures 
(9-10 in one segment), which connect the widely sep- 
arated longitudinal nerve cords. Two nerves proceding 
from the brain to tongue, pharynx and oesophagus, rep- 
resent a sympathetic nervous system. The two dorsal 
cephalic eyes correspond in their structure to the eyes 
of Alciopea. Ontogenetically it is an invagination of 
the cephalic ectoderm. The blood-vascular system of 
Peripatus is typical; the pericardial sinus is separated 
from the body cavity beneath by a horizonal, perforated 
septum. Every segment possess a pair of nephridia 
which exhibit the typical structure of those of the An- 
nulata. Salivary glands, anal glands and genital ducts 
are modified nephridia. Of extraordinary importance is 
the fact that Peripatus possesses the respiratory organs 
which are characteristic only of the Tracheata, namely 
tiachecz or air tubes, They consist of very long and thin 



ANTENNATA. 207 

chitinous tubes which united into tufts open at the base 
of flask-shaped depressions of the integument. Their 
number and arrangement vary. Coxal glands are present 
in all appendages except the first pair. They are very 
long in the male. The above-mentioned salivary glands 
are transformed coxal glands extending through the 
body. The sexes are separate. The female apparatus 
consists .of ovaria (attached to pericardial septum), uteri 
(much coiled), unpaired vagina (ventrally between 
penultimate legs); receptaculum seminis and receptacu- 
lum ovorum, each one attached to an uterus. The male 
apparatus consists of testes, vasa efferentia, seminal vesi- 
cles, vasa deferentia (much coiled) and ductus ejacula- 
torius, which secretes a spermatophore. Peripatus is 
viviparous. The eggs develop in the uterus, in which 
all stages are found, the youngest nearest the ovarium, 
the oldest nearest the vagina. 

The anatomic and ontogenetic relations show that 
Peripatus unites the characteristics of the Annulata and 
the Arthropoda, although those of the latter preponderate. 
Phylogenetically it must be considered as the central 
link of a chain which begins, with the Annulata and ends 
with the Insects. The Annelid characters of Peripatus 
are: the segmental nephridia, the segmental coxal glands, 
(setal glands of Chaetopoda), and the cutaneous muscu- 
lar envelope ; the Arthropodal characters are : the tra- 
cheae, the dorsal vessel and the lacunar system of circu- 
lation, the transformation of appendages into mouth 
parts (jaws), and the specific form of the salivary glands. 

A?itennata. The body of the Myriopoda consists of the 
head and a large number of equally developed trunk 
segments, the three anterior of which correspond to the 
three thoracic segments of the Hexapoda. The head is 



208 ARTHROPODA. 

the result of a fusion of at least four segments. The 
typical Antennate form Symphyla consists of twelve dis- 
tinct segments with ambulatory appendages and of an 
anal segment with two processes called spinning styles. 
The two preanal apparatuses of touch may indicate the 
transformed feet of a thirteenth segment, so that the 
whole number of segments corresponds to the original 
number of the hexapodal segments. In the Pauropoda 
there are only ten trunk segments, whilst in the Chilo- 
poda and Diplopoda there are many more (secondarily 
acquired). The body of the Hexapoda is divided into 
three distinct regions ; the unsegmented head (originally 
four segments), the thorax, consisting of prothorax, meso- 
thorax and metathorax and the abdomen with ten or eleven 
segments ; the winged insects have generally less than 
ten, owing to the fusion of either the penultimate or the 
first abdominal segments. In Macrolepidoptera, Diptera 
and Rhynchota the third thoracic segment approaches 
more closely the abdomen. The appendages of the in- 
sects consist of a single series of joints. We distinguish 
between the appendages of the head and those of the 
trunk. In the trunk only the three pairs of thoracic 
appendages are retained (rudiments of abdominal append- 
ages occur) The appendages of the head aie typically 
composed of four pair: antenna*, mandibles, anterior and 
posterior maxillcz. There is only one pair of preoral 
frontal antennae present, which vary greatly in form 
(in different sexes), and perform the functions of organs of 
touch and smell. 

The mouth appendages vary greatly according to their 
functions, but they can all be traced back to the mandi- 
bles, and the maxillae. The upper lip or labrum is a 
single piece and has nothing to do with the appendages, 



ANTENNATA. 209 

the mandibles (upper jaw) consist each of a strong but 
unsegmented masticatory plate with toothed margin . The 
anterior maxillce (lower jaw) consist each of a two-jointed 
basal part with five jointed palpus and two unsegmented 
jaws (mala externa and interna). The posterior maxillce 
form together the labium or underlip ; each one consists 
of a basal joint, a three-jointed palpus, and an external 
and internal jaw ; the basal joints have fused behind and 
beneath the mouth. There are numerous modifications 
which are indicated in the classification. The append- 
ages of the trunk assume manifold forms according to 
the various functions of walking, jumping, swimming, 
preying, etc. (Details in the classification.) Wi?igs are 
entirely wanting in the Myriopoda. The wingless con- 
dition of the Apterygota is typical as in the Myriopoda 
and Protracheata. Two pairs of wings represent the 
typical condition of the Pterygota, which may all be 
derived from a common ancestral winged group. The 
wings are appendages of the meso- and meta-thorax, 
being thin, lamellous, unsegmented reduplicatures of 
the body wall, especially of the integument. They 
are venate, and provided with nerves, branched tracheae 
and blood canals. The arrangement of these veins is 
of great importance for phylogeny and classification. 
The explanation of the origin of the wings is still 
a matter of conjecture. The integument and muscular 
structure are in the main the same as in the Crustacea. 
To be mentioned are especially the variation of integ- 
umental glands and the muscles of flight in the thorax. 
The alime?itary canal of the Myriopoda and Apterygota 
extends as a straight tube through the body ; in the 
winged insects it is more or less coiled. It consists in 
all cases of the distinct foregut (from the ectodermal 



2 1 ARTHROPODA . 

stomodaeum), the entodermal midgut and the hindgut 
(from the ectodermal proctodeum). Especially charac- 
teristic for most of the insects are the thread- or tube- 
shaped diverticula of the hindgut, which perform func- 
tions of excretion, and are known as the Malpighian 
vessels. Their large epithelial cells (with branched 
nuclei) contain urinary concretions. Sometimes they 
only consist of a few rows of cells. Their number is in- 
definite. The nervous system is essentially the same as 
that of the Crustacea, also the same modifications occur 
within different orders. Ocelli and compound eyes occur 
either together or separate. The ocelli of the Myriopoda 
are generally situated on the dorsal side of the head ; 
the larvae of the Hexapoda possess only ocelli, the adults 
show generally ocelli between the facetted eye, (See 
classification.) The structure of the compound eye is 
essentially the same as that of the Crustacean eye. They 
are divided intoeucone and acone eyes (see pages 57-59). 
The structure of the ocelli of the Dytiscus larvae and 
other insects and Myriopoda is worthy of special notice. 
It consists of a chitinous lens, beneath a hypodermal 
papilla whose basal cells form the retina; each retinal 
cell is connected with a nerve fibre, containing pigment 
and continuing externally into a rod; the cells along the 
margin of the papilla are free from pigment and form 
a glasslike body between retina and lens. On auditory 
organs see page 54. On the organs of smell, page 52. 
The system of blood circulation is very simple in the An- 
tennata. The contractile dorsal vessel (heart) is divided into 
successive segmental chambers (eight or nine) provided 
with valves which preveut the return of the blood from 
the anterior to the posterior chambers. Intersegmental 
paired ostia form an open communication between the 



ANTENNATA. 211 

body cavity and the interior of the heart. Triangular 
wing muscles (constant) are fastened with their broad 
end to the heart chambers, and constitute an incomplete 
horizontal partition wall, above the alimentary canal, 
thus aiding in the formation of a pericardial sinus. The 
heart is posteriorly blind, but continues anteriorly into an 
aorta which empties the blood into the lacunae of the 
body. Sometimes a ventral sinus is present, in which the 
flow of the blood is the reverse (also a heart nerve). 
A mass of large fat globules constitute the so-called fat 
body, which represents the nutritive reserve fund in meta- 
morphosis and reproduction. The luminous organ of 
certain beetles is a differentiation of the fat body under- 
going oxydation. 

The organs of respiratio?i are air-tubes or trachece, either 
opening tothe outside by paired, strictly segmental, ex- 
ternal apertures {stigmata) or anastomosing throughout 
the whole body and its appendages. The structure of 
the tracheae is everywhere essentially the same; it is in- 
ternally lined by a chitinous intima (a continuation of the 
external chitinous integument) which ends in a spiral 
thread to keep the air- tube open. An external cell layer, 
which secretes the intima, is a continuation of the exter- 
nal hypodermis. The modifications of the stigmata are 
manifold, adapted to the requirements for protection. 
The number of tracheae was originally the same as that 
of the segments, but reduction has greatly changed this 
condition. Respiration is brought about by the contrac- 
tion and expansion of the abdomen. Tracheal gills occur 
in the aquatic larvae of the insects. They are either ex- 
ternal (Odonata), consisting of three foliaceous gills on 
,the last abdominal segment or they are internal (Libellula, 
^Eschna), representing folds of the rectum. Other larval 



212 ARTHROPODA. 

gills are of various form and position. All these larvae 
are apneustic. The gills disappear with the larval stage, 
except in the Perlidae, iEschnidae, Sialidae, L,epidoptera, 
and Coleoptera. 

Special sounds are reproduced by certain insects, 
either by rapid movements of the wings, or of foliaceous 
trachaeal appendages (Hymenoptera, Diptera), or by the 
friction of uneven parts of the integument (Acridiidae, 
Gryllidae), or by special drum apparatuses in the first 
abdominal segment (Cicadae). 

All Antennata are of separate sex. The sexual organs 
are essentially the same as those of the Crustacea. In 
Peripatus the larger portion of the ducts is of ectodermal, 
in the Antennata of mesodermal origin. Exclusively 
paired sexual organs occur, however, only in the Ephe- 
meridse; in all other Antennata, fusions take place in 
various ways. The germ glands unite in the Myrio- 
poda and Diplopoda; unpaired terminal portions arise in 
all Antennata, except in the Ephemeridae and Diplopoda. 
Accessory sexual apparatus occur either as evaginations 
of the ductus ejaculatorius or of the paired vasa defer- 
entia, when they become (unpaired or paired) vesicula 
seminalis; accessory glands may secrete spermatophores. 
The terminal apparatus is often a protrusible penis; in 
the female it constitutes the bursa copulativa and the 
receptaculum seminis. They frequently open near the 
anus in a common cloaca. Special modifications of the 
last abdominal segments become parts of the sexual ap- 
paratus as ovipositors, stylets, etc. In most Diplopoda 
the legs of the seventh abdominal segment assume 
copulatory functions. 

Sexual dimorphism manifests itself in broad differences 
of male and female forms. The females of the Coccidae, 



antennata. 213 

and lyampyridae, of Psyche and Orgyia are without 
wings. The females of the parasitic Strepsiptera are 
viviparous, but remain maggot-like in the abdomen of 
their hosts. Polymorphism occurs in insects which form 
communities (bees, ants, white ants). 

Ontogeny of the Antennata. The embryonic develop- 
ment of the Insects takes place within the egg. The 
organism which leaves the egg possesses the typical 
structure of an Insect, namely a segmented body, an- 
tennae, mouth-apparatus, thoracic appendages, developed 
nervous, digestive and trachaeal systems, the dorsal vessel, 
the muscular structure, etc. It moves and feeds freely. 
It is called a larva. The larvae of the Insects are, there- 
fore, much more highly developed than those of most other 
invertebrate animals. The factors which condition the 
numerous changes which an insect larva undergoes before 
it reaches the adult stage {imago) are principally the 
degree of deviation from the original insect form and the 
difference in the habitat of larvae and imagines. Where 
development takes place without ??ieta?norphosis (ametabol- 
ism of Apterygota) there is no difference between larva 
and rtnago, except that the sexual organs are not fully 
matured in the young animal. In cases of gradual meta- 
morphosis the larva gradually develops wings by numer- 
ous molts of the integument (Orthoptera, Corrodentia, 
Thysanoptera, most Rhynchota). The larvae of the 
Cicada live in the earth, their imagines upon trees and 
bushes. In the transition a transformation of the an- 
terior legs takes place which necessitates an intermediate 
stage, the pupa, so that we have a gradual metamorphosis 
with pupa stage. The larvae of the Ephemeridae, Odonata 
and Plecoptera live in water, their imagines on land; 
during the transition the tracheal gills mostly disappear 



214 ArThropoda. 

and a holopneustic system of tracheae arises very gradu- 
ally; here incomplete metamorphosis (hemiruetabolism) 
takes place. In all these cases the larvae appear at first 
without wings; in some species of the Plerygota wings 
are never developed (reduction or acquired ametabolism) . 
All other Insects undergo complete metamorphosis (holo- 
metabolism). The larvae do not change at all, but pass 
over into the pupa stage, which is mostly quiescent. The 
pupae are very differently formed, often distinctly seg- 
mented with closely packed appendages and rudimentary 
wings, often with concealed appendages. When they 
are unable to procure food they are protected by special 
envelopes (e. g., cocoons). At the end of the pupa stage 
the envelope opens and the imago escapes. Complete 
metamorphosis arose from incomplete metamorphosis 
through the suppression of the frequent molts of the 
integument. These larvae are divided into two groups, 
those with feet (Neuroptera, Lepidoptera, Coleoptera, 
Trichoptera), and those without feet (apodal maggot- like 
Diptera, most Hymenoptera and Siphonaptera). The 
mode of living of these larvae varies greatly; they are 
originally all peripneustic, but become by adaptation 
amphipneustic, metapneustic, and even apneustic, devel- 
oping gills. Their mouth apparatuses may also be very 
different from those of the imago (I^epidoptera) . A great 
many modifications of the normal conditions occur (Apis, 
Sitaris, Pteromalidae). Internal metamorphosis manifests 
itself in the gradual development of the imago organs 
from corresponding larval organs, which gradually dis- 
integrate and disappear (function of phagocytae) The 
embryonic development of the Insects (p. 40) differs. from 
that of the Myriopoda in the formation of the amnion 
(internal) and the serosa envelope, an epithelial mem- 



ARACHNOIDEA. 215 

brane surrounding the whole egg. Both, however, have 
no part in the upbuilding of the embryo. The Myriopoda 
undergo a partial metamorphosis, inasmuch as the pos- 
terior segments in some larval forms (Scutigeridae, Litho- 
biidse, Chilopoda) increase until the required number of 
the respective imago is reached; the body differentiates 
from the anterior towards the posterior end (not observed 
in the Insects). 

The peculiar cases of parthenogenesis in certain Lepi- 
doptera and others, of heterogeny in the Aphides and 
Chermes abietes, and of pedogenesis in Cecidomyia, de- 
serve special mention. 

Phylogenetically the Insecta and Myriopoda are closely 
related, the Symphyla and Thysaneura forming the 
transition groups from the latter to the former, whilst 
the Myriopoda exhibit a close resemblance to Peripatus, 
so that Onychophora, Myriopoda and Insecta represent a 
phylogenetic series, which in Peripatus joins the hypo- 
thetic ancestral type of the Arthropoda (Protostraca) and 
through that the Annulata. 

Arachnoidea. The first seven (?) anterior segments 
unite into a cepalo-thorax (mostly unsegmented). 

The abdome?i generally consists of a variable number 
of separate or fused segments, but it may itself fuse with 
the cephalo thorax (Acarina, Linguatulidse) into one 
uniform structure. Thus a gradual concentration of the 
whole body is observed in the Arachnoidea. The Scor- 
pionidae and Solpugidse, representing the transition 
groups, possess the largest number of segments. The 
cephalo-thorax of the latter is divided into an anterior, 
unsegmented region, the head (resembling the Anten- 
nata), and into a posterior region, the thorax, consisting 
of three segments (as in the Antennata); this is followed 



2i 6 Arthropoda. 

by the abdomen with ten segments. The cephalo-thorax 
of the Scorpionidae is only segmented (seven) in the em- 
bryo. (See classification.) The Arachnoidea are typi- 
cally provided with six pairs of appendages which 
belong exclusively to the cephalo-thorax. The abdo- 
men is always without appendages. The first pair of 
appendages, the Cheliceia (mandibles, maxillary an- 
tennae, chelate antennae), is situated above and before 
the mouth. They are either three or. two-jointed. 
They are chelate when the claw-like terminal joint 
can be moved against a process of the preceding joint 
(Scorpions, many Acarina), or sub-chelate when the 
last joint is folded down upon the next like the blade 
of a pocket-knife (Spiders). The second pair of ap- 
pendages, the pedipalpi or maxillce are situated on 
both sides of the mouth, and mostly perform the func- 
tion of jaws. They consist of a stout basal-joint and a 
segmented palp, which may serve as organ of touch or 
acquire the form and segmentation of a leg. This either 
ends with or without a claw, or with a chela, and per- 
forms functions of prehension, or locomotion or copula- 
tion (see classification;. The other four pairs of append- 
ages are mostly of similar structure (six-jointed), and serve 
as organs of locomotion (third and fourth pair modified 
in Scorpions, so the third in Pedipalpi). 

The nervous system corresponds to the segmentation of 
the body, tending towards concentration. The brain is 
connected with the ventral cord by a short oesophageal 
commissure and innervates the chelicerae (different in 
Phalangidae and Gamasidae) and the eyes. Its ontogeny 
shows that it has fused with the first postoral ganglion 
pair of the embryo, which would indicate that the cheli- 
cerae are homologous with the mandibles of the Anten- 



ARACHNOIDEA. 2 1 7 

neta. The ganglia of the cephalo-thorax fuse together 
with a number of abdominal ganglia into a large thoracic 
ganglio7i mass, which sends nerves to the other five pairs 
of appendages and to the anterior abdominal segments ; 
the number of abdominal ganglia varies. The whole 
central nervous system of the dipneumonous Araneidea 
and of the Acarina is represented \>y one single fused 
nerve mass, perforated by the oesophagus and sending 
out radiating nerves. Most Arachnoidea possess eyes 
which are uniconical and of the same structure as the 
ocelli of the Antenneta. They are sessile, their number 
varying from two to twelve, symmetrically arranged 
upon the upper side of the cephalo-thorax. The struc- 
ture of the media?i eyes of ScorpionidcB corresponds to the 
ocellus in the possession of a singular cuticular corneal 
lens and to the facetted eye, inasmuch as its retina- cells 
are arranged in groups, as so-called retinulae. 

There are numerous glands opening on the external 
integument of the Arachnoidea. The spinning glands of 
the Araneidea open hy spinning mammillae (rudimentary 
abdominal appendages); preceding them is the so called 
cribrellum, a paired glandular region with numerous 
openings. There are also similar glands in other ap- 
pendages of the body (coxal glands, spinning glands, 
protrusible vesicles and the poison glands of the Aranei- 
dea). A great many cuticular glands (so the poison 
gland of the Scorpion) occur in almost all Arachnoidea. 
The alimentary canal consists of foregut (pharynx and 
oesophagus, the latter widening into the sucking stomach 
of the Araneidea), large midgut with blind diverticula 
(chylus stomach) and short hindgut into which tubular 
organs of exc7-etion open, corresponding to the Malpighian 
tubes of the Antennata. The blood-vascular system re- 



2l8 ARTHROPOD A. 

sembles that of the Antennata (highest in Scorpionidea 
and Araneidea). The heatt varies from the large, many- 
chambered dorsal vessel to the one-chambered heart sac 
of some mites. A pericardium is only rarely present. 
The most constant part is the aorta cephalica, a remnant 
of an original anterior part of the dorsal vessel. The 
heart of all Arachnoidea is situated in the abdomen. 
The orga?is of respiration are trachece whose external 
openings or stigmata (1-4 pairs) are almost exclusively 
situated on the anterior ventral part of the abdomen. 
They are tubular trachece (corresponding to those of the 
Antennata) ox fan trachece, generally called lungs, lung- 
tracheae, lung sacs or foliaceous tracheae (belonging ex- 
clusively to the Arachnoidea). Three modifications of 
the tubular tracheae occur. The chief branch proceding 
from the stigma may divide into many branches (Solpu- 
gidae, Cyphophthalmidae, Phalangidae, Pseudoscorpion- 
dae, Gamasidae, Ixodes), or only in two (Araneidea, 
Acarinaes), or it may be wanting altogether, the tracheal 
tubes proceeding directly from the stigma (Chernes 
cimicoides). The fan trachece consist of hollow lamellae 
placed upon one another in great number like the leaves 
of a book, and connected together by trabecul so as to 
have the form of a sac. They are always kept open by 
a firm internal chitinous membrane, so that the air can 
enter by the stigmata at the beginning of the abdomen 
and be distributed to the finest ramifications. 

All Arachnoidea are of separate sexes. The structure 
of the sexual organs varies greatty. The male organs 
consist of paired testicular tubes, and the vasa deferentia 
often receive the contents of accessory glands before 
opening to the exterior by a single or double aperture at 
the base of the abdomen. Special copulatory organs in 



ARACHNOIDEA. 219 

the region of the genital openings are, as a rule, want- 
ing, but appendages, far removed from the genital open- 
ings, often serve to transfer the sperm from the male to 
the female. The female sexual organs are also paired, 
generally racemose glands, with two oviducts, which 
usually dilate into a receptaculum seminis before their 
single or double opening at the beginning of the abdo- 
men. They are also connected with accessory glands; a 
protrusible ovipositor is rarely present. Only a few of 
the Arachnoidea are viviparous (Scorpions and some 
Mites); the greater number lay eggs which they some- 
times carry with them till they are hatched. The em- 
bryonic development exhibits a much more numerous 
metameric segmentation than we find in the adult ani- 
mal. The serosa and amnion membranes have only been 
observed in the Scorpions. The newly-hatched young 
have the form of the adult. Only in the Pseudoscorpion- 
idse and the Acarina a post-embryonic metamorphosis 
occurs (very complicated in Pygnogonida, Pentastomida 
and Hydrachnea — water mites). 

It is highly probable that the Arachnoidea arose 
together with the Pal seostraca from lower ancestral forms, 
but afterwards branched off, while the other Tracheata 
belong to another stem which is connected with that of 
the Arachnoidea at the base. 



220 



ARTHROPODA. 



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MOLLUSCA. 



Originally bilaterally symmetrical animals with un- 
segmented body. The dermal muscular structure of the 
lower, i. e., ventral surface, is greatly developed, and 
gives rise to a more or less projecting locomotory organ 
of very various shape, the foot. A reduplicature of the 
body wall forms a circular fold, the mantle which hangs 
over the mantle cavity. This cavity is originally most 
spacious at the posterior end, where it contains the two 
gills, symmetrically grouped on both sides of the median 
anus, the two nephridial openings and the sexual aper- 
tures. The dorsal portion usually develops into a visceral 
sac and is protected by a shell which extends to the edge 
of the mantle. The mouth is situated in the anterior 
region of the body and leads into a pharynx mostly 
provided with jaws and a hard chitinous plate, the 
radula. The midgut contains a voluminous digestive 
gland (liver.) The secondary body cavity is reduced, 
but always constitutes the pericardium. The blood - 
vascular system is open, largely lacunar ; the heart 
dorsal, originally with two symmetrical auricles, arterial. 
Nephridia originally paired, in open communication with 
the pericardium. The central nervous system consists 
of the paired cerebral, pleural, pedal and visceral ganglia. 
Of separate sexes or hermaphrodites. Gonads mostly 
unpaired, with paired or unpaired ducts. A modified 
trochophora arises from the gastrula, the characteristic 
Molluscan veliger larva. 



222 MOLLUSC A. 

This short general characteristic has to be modified 
for each class. Shell, mantle, gills, foot and oral arma- 
ture may disappear, and a displacement of the mantle 
organs may produce a far-reaching symmetry. 

I Class: Amphineura. Bilaterally symmmetrical 
Molluscs. The nervous system usually consists of two 
lateral and two ventral cords, connected by numerous 
commissures and supplied with ganglion cells; they join 
anteriorly the cerebral ganglion. Special sensory organs 
reduced. Marine. 

i. Order: Placophoia sive Chitonidcz. Eight calcareous 
plates, arranged successively like the tiles of a roof, cover 
the dorsal side. Separate cephalic region with mouth. 
Numerous gills on either side, in the furrow between the 
foot and the mantle zone. Foot (except in Chitonellus) 
strongly developed, with large, flat sole, adapted for 
crawling or clasping. Paired genital ducts and paired 
nephridia. Of separate sexes. Heart with two auricles. 
Radula (3-f 1), (2+1), (1 + 1 + 1), (1+2), (1+3)- Chiton. 
Chitonellus. 

2. Order: Aplacophora sive Sole?wgastres. Body almost 
cylindrical, usually worm-shaped. No shells. Calcareous 
spicules are embedded in the dense cuticula. Foot rudi- 
mentary. Mantle cavity reduced to a furrow on both 
sides of the rudimentary, septate foot, and a cavity 
(cloaca) at the posterior end, into which alimentary canal 
and nephridia open and where the rudimentary gills are 
situated, whenever such are present. The nephridia 
serve as genital ducts. 

1. Family: Neomeniida . Foot a longitudinal septum 
arising at the bottom of a medio-ventral longitudinal 
furrow. Hermaphrodites. Proneomenia. Neomenia. Lep- 
idomenia. Dondersia. 2. Family: Chcetodermida. Foot 



GASTEROPODA. 223 

and foot- furrow entirely reduced; sexes separate. Chceto- 
dermcE. 

II Class: Gasteropoda (Cephalophora). Snails. Body 
asymmetrical. Head, bearing tentacles and eyes, usually 
distinct from the body. Foot well developed, in most 
cases adapted for crawling. The large visceral sac tapers 
gradually and is usually spirally twisted (may disappear 
secondarily); it is covered by a single shell (case) into 
which the animal can withdraw. Wherever a secondary 
reduction of the visceral cavity takes place the shell may 
become rudimentary or disappear altogether (rarely in 
the Prosobranchia). Mantle-complexus upon the right 
(rarely left) side or moved (along this side) towards the 
anterior end. Visceral sac and shell spirally twisted. 
Asymmetry manifests itself in all groups (except the 
lowest Prosobranchia) in the disappearance of one gill, 
of one nephridial organ and of one auricle. 

1. Order: Prosobtanchia. Pleuro-visceral connective 
cross-striped. Mantle-complexus at the anterior region 
of the visceral sac. On most forms only one gill before 
the heart. Auricle before the ventricle. Animals of 
separate sexes, largely marine. Foot usually with oper- 
culum (cover) to close the shell. Only Titiscania with- 
out shell. 

1. Suborder: Diotocardia. Heart with two auricles 
(excl. Docoglossa). Two nephridial organs. Two gan- 
glionic longitudinal nerve cords in the foot, connected 
by numerous transverse commissures. Gills of pectinate 
form (double row) apex projecting freely. Epipodium 
well developed ; a circle of tentacles, varying in number, 
around the base. Without proboscis, penis and sipho. 

a. Zeugobranchia^Rhipidoglossa. Aspidobranchia). Two 
gills, both auricles well developed. Heart perforated by 



224 MOUvUSCA. 

the rectum. Shell with marginal slit, or with apical 
aperture, or perforated by a row of holes. Usually with- 
out operculum. Marine. Family: Haliotidce, radula oo 
i. (5. 1. 5.) 1. 00. Fissurellidce : Fissurella, rad. 00 1. 
(4. 1. 4.) 1. 00, with secondarily symmetrical shell. 
Emargi?iula. Scutum {Parmophoius .) Pleurotomaridce : 
Pleurotomaria. Scissurella. Polytremaria. Bellerophontida, 
fossil. 

b. Azygobranchia. One gill, the left of the Zeugobran- 
chia. Right auricle a blind diverticulum. Heart perforated 
by the rectum. Family: Turbonidce, rad. 00 o. (5. 1. 5-)o. 

00. Trochidce. Stomatiidcs. Neritopsidce, rad. co 1. (2.0. 2.) 

1. co, marine. Neritidce, rad. 00 1. (3. 1. 3.) 1. 00, 
marine, are capable of living out of water along the coast. 
Neritintz, fresh water forms. Hydrocoenidcs, rad. 00 1. 
(1. 1. 1.) 1. co, and HelicinidcB, 00 1. (4. 1. 4.) 1. 00 , 
are both without gills, and possess a lung similar to that 
of Pulmonata. The Helicinidse are terrestrial. 

c. Docoglossa. Heart with one auricle, not perforated 
by the rectum. Left nephridial organ moved to the right 
of the pericardium. Visceral sac and shell secondarily 
symmetrical, the latter usually cup-shaped. Operculum 
wanting. Marine. 

1. Left true gill (Ctenidium) present. Acmcuzdce, 
rad. 1. 2. (1. o. 1.) 2. 1, with numerous accessory gills 
in the mantle furrow: Scurf ia ; without such gills: 
Acmcea (Tecturd). 

2. True gills (Ctenidia) wanting altogether, acces- 
sory gills present in large numbers in the mantle furrow. 
Family: Patellidce, rad. 3. 1. (2. o. 2.) 1. 3. 3. Without 
ctenidia and accessory gills. Lepetidce ; rad. 2. o. 1. o. 2. 

2. Suborder: Monotocardia (Pecti?iibranchia .) Heart 
with one auricle. One single true gill, of pectinate 



GASTEROPODA. 225 

form (single row), its apex not freely projecting (exc. 
Valvata). Usually pedal ganglia, rarely pedal cords. 
One nephridial organ. Sipho and penis present in most 
cases. Epipodium feebly developed or wanting. Largely 
marine snails of very variable shape. 

a. Architccnioglossa. Pedal cords. In Cypraea (and 
other forms) a rudiment of the right auricle still present. 
Family: Gyp resides, rad. 3. 1. 1. 1. 3. Paludinido: (fresh 
water). Cyclophorides (terrestrial, with lungs). 

b. Tceiiioglossa. Typical radula 2. 1. 1. 1. 2. Semi- 
proboscidifera: Families: Naticidae. Lamellaridae. Ros- 
trifera: Families: Yalvatidae (fresh water). Ampullaridae 
(freshwater). Littorinidae. Cyclostomidae (terrestrial). 
Planaxidae. Hydrobiidae (fresh water). Aciculidae (ter- 
restrial). Truncatellidae (partly terrestrial). Hyppo- 
nycidae. Capulidae. Calyptraeidae. T'seudornelanidae. 
Melanidse. Cerithiidae. Yermetidae. Turritellidae. 
Xenophoridae. Struthiolaridae. Chenopidae. Strom- 
bidae. P7-oboscidifera holostomata. . Families: Scalaridae, 
rad. n, O, n. Solaridae, rad. n, O, n. Pyramidellidae, 
rad. O. Eulimidas, rad. O. Proboscifera siphonostomata. 
Families: Colombellinidae. Tritoniidae. Cassidiidae. 
Doliidae. Ianthinidcz, rad. n, O, n. Heteropoda, (pel- 
agic Taenioglossa with fin-like foot (perpendicular). 
Families: Atlcmtidcz. Pterotracheidce . 

c. Ste?wglossa. Normal rad. 1. 1. 1. Rachiglossa. 
Families: Turbinellidae. Fusidae. Mitridae. Buccinidae. 
Muricidae. Purpuridae. Haliadeae. Cancellaridae. Vol- 
utidae. Olividae. Marginellidae. Harpidae. Toxiglossa. 
Families: Pleurotomidae. Terebridae. Conidae. 

2. Order: Pulmonata (Lung-snails). Pleuro -visceral 
connective, not cruciform. Gill displaced by a respiratory 
vascular network on the interior surface of the mantle 



226 MOUJJSCA. 

(lung). Pallial complexus originally on the anterior 
right side of the visceral sac. Margin of the mantle fused 
with the cervical integument, leaving only a respiratory 
aperture on the right side. Visceral sac and shell fre- 
quently rudimentary in terrestrial forms (night snails). 
Operculum usually wanting. Heart with one auricle, 
generally in front of the ventricle. Hermaphrodites 
with hermaphroditic gland and complicated eductory 
apparatus. Terrestrial and fresh water animals. 

i. Suborder: Basommatophora (fresh-water forms). 
Eyes at the base of the ocular tentacles (not invaginable). 
Sexual apertures separate on the right anterior region, 
male in front of female. Family: Limnceidce. I^imnsea. 
Amphipeplea. Physa. Planorbis. Ancylus. Auricu- 
UdcE. 

2. Suborder: Stylommatopho') a. Byes at the apex of 
the ocular tentacles. Tentacles invaginable. 

a Monogonopora. With a single sexual aperture 
(to the right). Family: HelicidcB. Helix. Arion. 
Bulimus. Testacellidtz : Daudebardia. Testacella. Lima- 
cidce : Arjophanta. I^imax. Vitrina. Zonites. Heli- 
carion. BulimulidcE. Pupidcz. Buliminus. Pupa. 
Clausilia. Succineidce. 

b Digonopora. Nocturnal snails with separate male 
and female sexual apertures. Both on the right side, male 
at the anterior, female at the posterior end. Palleal 
complexus at the posterior region, lung-cavity reduced. 
Families: Vaginulidcs (terrestrial). Oncidiidse (marine 
or amphibious), respiration partly through respiratory 
dorsal appendages. 

3. Order: Ophistobranchiata. Pleuro- visceral connec- 
tion not cruciform. One auricle behind the ventricle. 
Hermaphrodites. Shell present or (more frequently) 



GASTEROPODA. 227 

wanting. Operculum usually wanting. Respiration by 
genuine ctenidia or adaptive gills or by the integument. 
Visceral sac very often reduced. Hermaphrodites with 
hermaphroditic gland. Marine. 

1. Suborder: Tedibranchiata. Pallial complexus on 
the right side, more or less covered by the mantle fold. 
Always one gill (originally the left) present, only in- 
completely covered by the mantle. Visceral sac tending 
to reduction. Shell always present, tending to become 
rudimentary. Usually with parapodia and mantle lobes 
covering the shell. 

I. Reptantia: (a) Cephalaspidea, with frontal disc. 
Families: ActceonidtE (with operculum). ScaphandridcE . 
Bullidce: Bulla. Acera. Gasteropteridce . PhilinidcE. 
DoridiidcE. (b) Anaspidea. Head without frontal disc. 
Four lobe-like or auricular tentacles. Family: Aplysii- 
dcB. Aplysia. Dolabella. Notarchus. (c) Notaspidea. 
Head short, with or without tentacles. Dorsal region 
forms a large disc (notaeum) upon which a shell may lie. 
Families: PleurobranchidcE : Pleurobranchus. Pleurobran- 
chaea. Oscanius. Umbrellidce: Umbrella. Tylodina. 
Pel tides. 

II. Nata?itia sive Pteropoda (fin-snails). This group 
constituted formerly a special class of Mollusca, but is 
now classified as Tetrabranchiata, adapted to the free 
swimming pelagic mode of life. The parapodia of the 
Tectibranchia are developed into fins or wing- shaped 
swimming organs, (a) Ptetofroda thecosomata. Shelled 
fin-snails. More closely related to the Cephalaspidea. 
Mantle, mantle- cavity, shell present. Head not distinct. 
Only one pair of tentacles. Fins fused along their an- 
terior margin above the mouth. Anus on the left side. 
Families: Limacinidce. External calcareous shell twisted 



2 28 MOLLUSC A. 

to the left with a spiral operculum. Anus on the right 
side. Iyimacina. Peraclis. Family: Cavoli?iiid<z . Ex- 
ternal calcareous shell symmetrical. Clio. Cavolinia. 
Family: Cymbuliidce. Internal cartilaginous shell. Cym- 
bulia. Cymbuliopsis. Gleba. The Thecosomata feed 
largely on small Protozoa and Algae, (b) Pteropoda 
gymnosomata. Naked fin-snails. They are more closely 
related to the Anaspidea. Without mantle, mantle 
cavity and shell. Head distinct. Two pairs of tentacles. 
Fins separate. Anus on the right side. Family: Pneu- 
modermatidce. Ctenidium on the right side. Dexio- 
branchsea. Spongiobranchsea. Pneumoderma. In the 
two last genera an additional adaptive posterior gill. 
Families: Clionopsidcs and Notobranch<zid<z . No Cteni- 
dium. An adaptive posterior gill. Family: ClionidcE. 
Neither ctenidium nor adaptive gills present. All Gym- 
nosomataare predatory, feeding largely on Thecosomata. 

2. Suborder: Ascoglosea. The used up teeth of the 
long and narrow radula (consisting of a single row of 
plates) preserved in a pouch at the anterior end of the 
radula. No jaws. Anus almost always dorsal. Cten- 
idium, mantle and mantle cavity absent (except in Steg- 
anobranchia). 

i. Section: Steganobranchia. With mantle on the 
right side, mantle cavity, ctenidium, shell and parapo- 
dia. Family : Oxynoidece: Oxynoe. I^obiger. 

2. Section : Cirrobranchia . Along the sides of the 
dorsal region foliacious or club-shaped processes. Fam- 
ilies: Herm<zid<z. Phyllobranchidce . 

3. Section: Pterobranchia . Lateral parts of the body 
drawn out into lobes. The branches of the midgut gland 
(liver) extend into these lobes. Families: Elysiadce. 
Placobranchidce \ 



GASTEROPODA. 229 

4. Section: Abranchia. Neither ctenidium, nor dorsal 
appendages, nor foliacious lateral extensions of the body. 
Integnmental respiration.. Body almost planaria-like. 
Family: L imapo?i Hides . 

3. Suborder: Nudibra?ichia. Without mantle fold, 
shell and ctenidium. Jaws usually present. Radula 
mostly well developed, with teeth which may disappear, 
Adaptive gills very variably developed, sometimes O. 

1. Section : Holohepatica . A large compact (un- 
branched) digestive gland (liver.) Numerous gill lam- 
ellae in a furrow around the body. Without jaw and 
radula. Pharynx transformed into a suctorial apparatus. 
Cells form a rosette around the dorsal anus. Dorididcc 
cryptobranchiatce . With gill-rosette around the dorsal 
anus, retractable into a cavity. Bathydoris. Archi- 
doris. Discodoris. Dianlula. Kentrodoris. Platy- 
doris. Chromodoris, etc. Dorididce pha7ierobra?ichiatcE . 
Gill- rosette not retractable. Goniodoris. Polycera. 
Acanthodoris. Atalia. Ancula. Euplocamus. Tri- 
opa, etc. 

2. Section: Cladohepatica. lyiver entirely or partly 
dissolved into branched, separate canals, extending far 
into the body. Dorsal appendages of various form 
usually with respiratory functions. Anus usually on 
the right side. Families: AZolidiadcE: iEolidea. Berghia. 
Tergipes. Galvina. Coryptella. Rizzolia. Facellina. 
Flabellina. Fiona. Glaucus. Janus. Hero. Tethy- 
melibidce, without radula : Tethys. Melibe. Lomano- 
tid<z. DotonidcE. Dendronotidcu. Bornellida. Scyllce- 
idce. Phyllirhoidce . Pelagic free swimming animals, 
with narrow laterally compressed body, without foot and 
without respiratory appendages. Pleurophyllidiidce , nu- 
merous gill lamellae arranged in a longitudinal row in a 



230 MOIXUSCA. 

furrow on either side between dorsal shield and foot. 
Pleuroleuridce : Tritoniadce . Tritonia. Marionia. 

III Class: Scaphopoda. Body symmetrical, length- 
ened in a dorso- ventral direction. Mantle a tubiform 
sac with a more narrow, dorsal, and a wider ventral 
opening. Mantle cavity extending at the posterior 
region as far as the apical aperture. Shell high, shaped 
like a conical tube, with a smaller apical and a larger 
ventral opening. Ctenidia wanting. Nephridia paired. 
Blood-vascular system (independent) reduced to one 
ventricle without auricles. Sexes separate. Special 
genital ducts wanting. The right nephridium performs 
their function. Mouth at the end of a head-like buccal 
prolongation, and surrounded by a circle of foliaceous 
appendages. At the base of the head-like prolongation, 
numerous filamentous appendages arise which are pro- 
trusible through the lower opening of the shell and the 
mantle. Foot stretched, prolonged ventrally. Radula 
present. Iyiniicolous marine forms. Dentalium. Foot 
comparatively short, at the end almost acorn- shaped, 
with a conical middle and two lateral lobes. Siphono- 
dentalium. Foot worm-like, lengthened, at the end wid- 
ening into a disc, with papillae along its margin. 

IV Class: Lamellibranchia (Pelecypoda, Bivalva, 
Acephala, Aglossa). Body symmetrical, transversely, 
more or less flattened, with two large, lateral, mantle 
lobes covering a spacious mantle cavity, within which 
the hatchet- or club-shaped foot may be withdrawn. Two 
lateral valves, only connected at the dorsal hinge margin. 
Either with two transverse adductory muscles (Dimyaria), 
or with only one (anterior one reduced, Monomyaria). 
On either side of the mantle cavity a ctenidium. With- 
out pharynx, jaw, radula, and tentacles — no separate 



SCAPHOPODA. 231 

cephalic region. Nephridia paired; genital organs 
paired, open by separate apertures or by means of nephri- 
dia. Heart with two auricles. On either side of the 
mouth a pair of labial palpi. Hermaphrodites or of sep- 
arate sexes. Marine and fresh water forms. Limicolous 
or sessile. 

1. Order : Protobranchia. Gill in the posterior part of 
the mantle cavity, bipectinate, corresponding to the 
ctenidium of the Zeugobranchia, projecting with its 
free apex posteriorly into the mantle cavity. Foot adapted 
for crawling. Pleural ganglion distinct from cerebal 
ganglion. Family: Nuculidce: Nucula. Ledia. Yoldia. 
SolenomyidcE. 

2. Order: Filibranchia . The gill leaves of the cteni- 
dium lengthened into long filaments, which hang down 
far into the mantle cavity and consist of two bars, a basal 
descending and a terminal ascending one. Family : 
Anomiidce. Mantle open without siphons : Monomyaria. 
Foot small. Body and shell asymmetrical. Sessile 
mussels. Gill threads entirely free. Anomia. Placuna. 
Family: Arcidce. Gill filaments of each series connected 
by ciliary discs. Dimyaria. No siphons. Large foot. 
Area. Pectunculus. Family : Trigoniidcz . Gills the 
same as in the Arcidae. Dimyaria. No siphons. Trig- 
onia. Family: Mytilidce (except A viculidae). Gill fila- 
ments connected by non- vascular sutures. Anterior 
adductory muscle smaller than the posterior one. (Het- 
eromyaria). Siphons present. Foot stretched. Mytilus. 
Modiola. Lithodomus (boring mussels). Modiolaria. 

3. Order : Psendolamellibranchia. The successive gill 
filaments of one series are connected by ciliary discs or 
by vascular bridges, so likewise each ascending filament 
with the corresponding- descending. Family : Pectenidcz, 



232 MOLLUSCA. 

Monomyaria with entirely open mantle supplied with 
eyes along its margins. Without siphons. Foot small, 
tongue-shaped. Shell with equal or unequal valves. 
Swimming. Peeten. Chlamys. Family : Aviculidce. 
Monomyaria or Heteromyaria without siphons. Valves 
either equal or unequal. Avicula (Meleagrina). Mal- 
leus. Vulsella. Perna. Inoceramus. Pinna. Melea- 
grina. Margaritifera (pearl mussel). Family: Ostreidce. 
Monomyaria without foot with entirely open mantle 
without siphons. Valves unequal, fastened by the left 
valve to a support. Ostrea (oyster). 

4. Order : Eulamellibranchia. Gills not consisting 
of distinct filaments. The filaments of each series and 
the two bars of a filament are connected by vascular 
bridges or sutures in such a way that each filamentous 
series resembles a sieve-like lamella. Thus on either 
side two such lamellae exist, which in reality correspond 
to the two rows of plates of a simple bipectinate cten- 
idium. Here belong the majority of Lamellibranchia. 

1. Suborder: Submytilacea. Gill lamellae smooth. 
Mantle usually fused only between the incurrent and 
excurrent apertures. Dimyaria. Family : Carditida. 
Dimyaria with open mantle and large foot. Cardita. 
Venericardia. Family: Lucinidce with simple siphonal 
openings of the mantle. Foot often vermiform. Fam- 
ily : Erycinidce. Mantle closed except along the two 
siphonal and the foot openings. Foot long Brycina. 
Kellya. Lasaea. Lepton. Galeomma. Family: Cras- 
satellidcE. Mantle open, without siphons. Family: Cy- 
renidce. Mantle open. Two siphons. Foot large. In 
fresh or brackish water. Cyrena. Corbicula. Sphaer- 
ium. Pisidium. Galatea. Family: Dreissensiidcz, in 
rivers. Family: Unionidce, in fresh water. Foot large, 



SCAPHOPODA. 233 

hatchet- or club-shaped. Two simple siphonal openings 
or slits. Mantle open. Unio. Anadonta. Mutela. 

2. Suborder: Telli?iacea. Dimyaria with entirely sep- 
arate siphons. Foot large. Gills smooth. Family: 
Tellinidcc : Tellina. Family: Donicidce : Donax. Mac- 
tridce : Mactra. 

3. Suborder: Veneracea. Dimyaria. Gill-lamellae 
somewhat folded. Siphons separate. Foot rather large. 
Family: Venerida in Venus. Meretrix (Cytherea). 
Tapes. Family: Petricolidce (boring mussels). 

4. Suborder: Cardiacea. Dimyaria or Monomyaria. 
Gill-lamellae highly folded. Mantle with two siphonal 
and one pedal opening, otherwise fused. Famil}^: Car- 
diidce. Dimyaria. Cardium. Family: Chamidce. Dim- 
yaria. Valves unequal. Chama. Diceras. Requienia. 
Here, probably, belong the fossil Monopleuridae, Capri- 
nidae, Hippuritidae, Radiolitidae. Family: Tridacnidcs. 
Monomyaria. Tridacna. Hippopus. 

5. Suborder: Myacea. Dimyaria with folded gill 
lamellae. Mantle with tendency towards fusion. Siphons 
very long. Foot large. Family: Psammobiidce . Pectal 
aperture of the mantle still very large. (Psammobia.) 
Family: Mesodesmatidcz . Lidrariidce . Myiidce. Mya. 
Corbula. Glycymeridce. Glycymeris. Saxicava (boring 
mussels). Solenidcc. Shell gaping at both ends. Foot 
very large. Solenocurtus. Cultellus. Ensis. Solen. 

6. Suborder: Pholodacea. Dimyaria with fused mantle 
and well-developed siphons. Foot variable, sometimes 
rudimentary. Shell gaping, frequently with accessory 
pieces. Family: Pholadidce. Boring mussels. Pholas. 
Pholadidea. Jouannetia. Xilophaga. Family: Tere- 
dinidce, boring mussels. Teredo (so-called Ship-worm). 



234 MOLL USC A. 

Family: Clavagellidce . Clavagella. Brechites (Asper- 
gillum). 

3. Suborder: A?iati?iacea. Mantle extensively fused. 
With siphons. Hermaphrodites. Foot present. Family: 
Pandoridce. Lyonsiidcs. Anatinidte. Anatina. Thracia. 

5. Order: Septibranchia. The gill is transformed on 
either side into a muscular, perforated septum, which 
divides the mantle cavity into two chambers, one above 
the other. Hermaphrodites. Families: Poromyidcs. Cus- 
pidaridce. 

V Class : Cephalapoda. Body symmetrical, with 
high visceral sac. Around the mouth tentacles or clasp- 
ing arms, considered to be parts of the foot which have 
grown anteriorly around the mouth. Another part of 
the foot is the funnel. In the posterior mantle cavity 
two or four ctenidia. Heart with two or four auricles; 
two or four nephridia. Unpaired gonads with paired or 
unpaired duct. Sensory organs, especially the eyes at 
the sides of the cephalic foot, highly developed. Strong 
jaws and firm radula. With external or internal shell, 
or without shell. Usually with ink-bag. L,arge, pred- 
atory marine animals of separate sexes. 

1. Order: Tetrabranchiata . With external chambered 
shell, in whose last (largest) chamber the animal dwells. 
Shell symmetrical, exogastrically coiled up. Numerous 
tentacles without suckers and retractable into special 
sheaths arise upon larger lobes around the mouth. 
Four gills, four auricles, four nephridial organs. Funnel 
consists of two lateral, separate lobes, which, with their 
free edges reaching one above the other, form a tube. 
Without ink-sac. With concave eyes. Only living 
form Nautilus. Radula 2. 2. 1. 2. 2. The two large 
groups of the Nautiloidea and Ammo?iitidea extinct. 



CEPHALAPODA. 235 

2. Order: Dibranchiata . Shell internal or rudiment- 
ary, or wanting. • Rarely (entogastrically) coiled. Two 
gills. Two auricles ; two nephridia. Eight or ten 
clasping arms with suckers around the mouth. The two 
lobes of the funnel fused along the free margin. Vesi- 
cular eyes. With ink-sac. 

1. Suborder: Decapoda. With internal, often rudi- 
mentary shell. With ten arms, the fourth pair of which 
is developed into long prehensile tentacles, retractable 
into special cephalic cavities. Good swimmers with 
dorso-ventrally elongated body provided with lateral fins. 
Genital ducts unpaired. Family: Spirulidce. With in- 
ternal, spirally twisted, entogastrically coiled shell. 
Spirula. Family : Belemnitidcz. Fossile forms with 
internal chambered, generally straight shell. Belem- 
nites. Spirulirostra. Belemnotenthis. Family: Oigop- 
sidcB: Ommastrephes, rad. 3. 1. 3. L,oligopsis. Cran- 
chia. Chirothenthis. Owenia. Thysanotenthis. Ony- 
chotenthis. Ommatostrephes. Family : Myopsidcc : 
Rossia. Sepiola. Sepiodarium. Idiosepion. Loligo. 
Sepiotenthis. Belosepia (fossil). Sepia, rad. 5. 1. 3. 

2. Suborder: Octopoda. Without shell; with eight 
arms; without prehensile tentacles. Body plump, usually 
without fins, little adapted to swimming. Genital ducts 
paired. Family: Cirrhotenthidce . With fins. Family: 
Philonexidcc. Argonatua, female with external uncham- 
bered shell. Philonexis. Tremoctopus. Family: Odo- 
pedidcE. Octopus, rad. 1. 3. 1. Eledone. 

Appendix: Rhodope Veranii. A small animal, 4 
mm. long, spindle shaped, externally bilaterally sym- 
metrical. Body epithelium ciliated; with cutaneous 
muscular envelope, in which calcareous particles are 
embedded. Alimentary canal without radula, jaws and 



236 MOIAUSCA. 

liver. Nervous system with two supra- and one infra- 
cesopriageal ganglion, with transverse commissures and 
connections, with eyes and auditory vesicles. Longitu- 
dinal cords. Hermaphrodite with complicated genital 
apparatus. No separate blood system. One spacious, 
ciliated, nephridial chamber. Development direct; no shell 
formation. Its systematic position still disputed. 

Organization of the Mollusca. The body of a primitive 
Molluscan type is bilaterally symmetrical with a convex 
dorsal region; the anterior region, containing mouth, 
eyes and tentacles, constitutes a distinct part of the 
body, the head. The ventral region is distinguished by 
a separate strong muscular plate, the foot, adapted for 
crawling. The soft integument of the convex dorsal 
region produces on all sides a large, overhanging fold, 
the mantle {pallium), which covers a circular cavity, the 
mantle cavity surrounding the trunk and communicating 
through the free margin of the mantle with the medium 
in which the animal lives. The dorsal integument of 
the trunk and its continuation, the external integument 
of the mantle secretes a closely joiniifg shell consisting 
of a chitinous matrix (conchyliolin), in which carbonate 
of lime is deposited. This shell is like the dorsal region, 
bilaterally symmetrical, convex. Separated from the 
body it would resemble a cup or plate. It serves both 
as a protection to the dorsal region and as a skeleton to 
which the muscles of foot and head are attached (dorso- 
ventrally directed). The mantle does not only form and 
enlarge the shell by its marginal secretions, but also 
covers the delicate gills and procures for them the pro- 
tection of the shell. We have a similar mechanism in 
the carapace of higher Crustacea and the branchial oper- 
culum of the fish, In the Mollusca the relations between 



MCMUSCA. 237 

gills, mantle, and shell are of the utmost importance. 
The gills, situated in the mantle cavity, are paired and 
symmetrical, there may have been originally a series of 
successive gills or only two on either side of the pos- 
terior region of the mantle cavity. Bach gill is shaped 
like a feather, consisting of a central axis or shaft and a 
double row of numerous (barb-like) side branches. The 
shaft rises free from the trunk into the mantle cavity. 
Near the base of each gill there lies a sensory organ (of 
smell), called the osphradinm. Such a gill has a definite 
morpholigical value. It has been termed a ctenidium, in 
order to distinguish it from analogous respiratory organs, 
occurring in certain Molluscs. The head bears a pair of 
tentacles and a pair of eyes. The mouth is situated at its 
anterior ventral side; the apertures of the internal organs 
at the posterior end of the trunk above the foot. Here, 
also, is found in the median line the anus, and on either 
side between it and each ctenidium (if we accept only 
one pair) two openings occur, one for the sexual organs 
and one for the nephridium. All these apertures (two 
ctenidia, two osphridia, anus, paired sexual and nephri- 
dial openings) are covered by the mantle and, therefore, 
lie in the mantle cavity. These parts constitute together 
the complex of pallial organs. 

The deviations from the primitive external organiza- 
tion as given above, have largely been indicated under 
the different classes and orders. In the Placophora the 
contours of the foot with a cephalic region in front of it, 
run parallel to those of the body. The mantle is rep- 
resented by the peripheral region between the margin 
of the body and the calcareous plates. It presents 
numerous scattered spines, which are sometimes hard 
and chitinous and sometimes calcified, arising in 



238 MOlvI,USCA. 

special follicles lined by ectoderm cells. Between 
the mantle and the foot a furrow (mantle cavity) 
occurs containing on either side a series of ^af-like 
gills, which may either form an almost complete 
circle of gills around the foot or be confined to the pos- 
terior region. Eyes and tentacles are wanting. The 
body of the Gasteropoda exhibits a multitude of modifica- 
tions ; it is either bilaterally symmetrical or highly 
asymmetrical. The same is true of the whole external 
structure. Usually the dorsal region of the body con- 
taining the viscera assumes a constricted sac-like form 
which, for the sake of reducing surface extension, coils 
itself spirally and forms a corresponding shell into 
which the protrusible head and foot can be withdrawn. 
The integument of the viscera sac may form along any 
place of the body an overhanging mantle fold, protecting 
the gills and aiding in the formation of the shell. In 
the Prosobranchia the pallial complex is not situated at 
the posterior, but at the anterior region of the body. 
The originally paired organs become (usually) unpaired 
and thus the asymmetry very prominent. They are 
called Prosobranchia because the gills are situated before 
the heart, whilst in the Opistobranchia the reverse is the 
case ; the pallial margin of the latter never forms a dis- 
tinct siphon. The mantle cavity of the Pulmonata is 
filled with air (gills wanting), respiration being carried 
on by the vascular network, covering the internal sur- 
face of the mantle fold immediately in front of the heart. 
At the base of the head-prolongation of the Scaphopoda 
two tassel-shaped masses of long filiform contractile ten- 
tacles arise which hang down into the mantle cavity, 
and can be protruded through the ventral mantle open- 
ing. The anus occupies a median posterior position 



MOIXUSCA. 239 

above the foot with a nephridial opening on either side. 
The mantle fold of the Lamellibranchia is attached to the 
trunk along its whole base ; it encloses a cavity whose 
transverse diameter is very much shorter than the dorso- 
ventral or longitudinal, i. e., the whole body is com- 
pressed. The two valves of the shell articulate at the 
dorsal region and gape along the ventral margin . The two 
adductor muscles (anterior and posterior) which trans- 
versely connect the two valves, produce distinct impres- 
sions on their inner surface. The mouth is situated 
between the anterior adductor and the anterior base of 
the foot ; the anus (frequently fringed) behind the pos- 
terior adductor. A distinct cephalic region is wanting. 
Along the line of the pedal insertion, on either side of 
the median and posterior part of the trunk a longitudi- 
nal septum (shaft) arises to which are attached numer- 
ous long gill-leaves, arranged in two rows, i. e., one gill 
lies on either side of the mantle cavity. It is of great 
importance to observe in the different groups of Lamelli- 
branchiata that the mantle can partially close itself 
whenever the free margin of the right fold fuses with 
the free margin of the left fold at one or more than one 
place. This leads to the formation of siphons which 
are contractile and extensible, governed by muscles. 
The body of the Cephalopoda is morphologically so ar- 
ranged that the apex of the large visceral sac accupies 
the highest point of the dorsal region (apparently the 
posterior end), whilst the head (transformed into a 
cephalic foot) with its prehensile anus occupies the cor- 
responding ventral position (apparently anterior end.) 
The morphologically anterior side is apparently above, 
the posterior below. (Compare the position of a Sepia in 
water.) The mantle fold, hanging down posteriorly from 



240 MOLLUSC A. 

the visceral sac covers a spacious mantle or branchial 
cavity which communicates above the cephalic foot 
through the mantle aperture with the external world. 
On the floor of the mantle cavity two or four symmetri- 
cally arranged gills are situated. In the posterior lower 
side of the visceral sac two symmetrically shaped lobes 
arise, approaching each otjier in such a way as to form a 
tube, the so-called funnel, whose one opening lies in the 
mantle cavity while the other appears outside of it, be- 
neath the pallial aperture. The water of respiration 
flows from the mantle cavity into the funnel and thence 
to the outside. The funnel is also used as an eductory 
passage by the faecal masses, the excretions, the genital 
products and the secretions of the ink-sac. Originally 
all Cephalopoda very probably possessed a complete shell 
covering the visceral sac and the mantle fold. Its mod- 
ern form is either rudimentary or wanting altogether. 
The vacant chambers of the shell of the Tetrabranchiata 
are filled with gas ; partitions separating them are per- 
forated in the center by a siphon which traverses all 
chambers and is fastened to the visceral sac of the ani- 
mal (Nautilus). That part of the foot which surrounds 
the mouth is drawn out into numerous tentacles, each 
one of which can be retracted into a special sheath ; 
the anterior end situated before and above the head is 
widened into a concave lobe, the so-called cephalic cap, 
which lies external to the anterior part of the dwelling 
chamber, closing the shell when the tentacles are with- 
drawn. The cephalic cap bears two tentacles. On either 
side of the head one eye is located. The mantle fold 
extends above the cephalic foot around the whole body. 
Whilst it is short along the sides, it forms a considera- 
ble lobe in front and above, and covers posteriorly a very 



MOLLUSC A. 241 

deep cavity. The funnel consists of two entirely sepa- 
rate lateral lobes (epipodial lobes) representing a part 
of the foot. Within the mantle cavity (upon the visceral 
sac) two pairs of pectinate gills (upper and lower) arise. 
Here also nine apertures occur : the median anal, two 
genital, two nephridial and two viscero-pericardial. The 
mantle fold of the Dibranchiata extends as a narrow 
margin along the lateral regions and the anterior side of 
the visceral sac, forming immediately above the cephalic 
foot a furrow, whilst it covers almost the entire posterior 
surface of the visceral sac, producing a very deep and 
spacious mantle cavity. The two lateral lobes of the 
funnel are fused along their free margins into a tube 
open on both ends; near the upper end the openings of 
the internal organs appear. The two gills are situated, 
one on the right and one on the left side in the mantle 
cavity. 

The whole Molluscan body is covered by a one-layered 
epithelium, which is usually ciliated in all parts, not 
covered by the shell. It is very rich in glands of the 
unicellular type, situated either in the epithelium itself 
or in the subjacent tissue with their ducts between the 
epithelial cells. The tissue immediately beneath the 
epithelium (connective tissue, muscular fibres) is distin- 
guished as the leathery integument or cutis, but there is 
no sharp line of separation between this and the tissues 
beneath it. In the cutis of the Cephalopoda large 
chromatophores occur which through alternating contrac- 
tions and expansions produce the beautiful change of 
color. Likewise the free pallial margin of the Gastero- 
poda from which the formation of the shell proceeds, 
contains numerous mucous chromatous and calcareous 
glands. The shell of the L,amellibranchiata consists of 
L 



242 MOU.USCA. 

three lamina: the external layer (horny cuticula), the 
intermediate (porcellan layer), consisting of perpendicu- 
lar calcareous prisms, and the internal layer or mother 
of pearl, composed of five calcareous plates, arranged in 
delicate wave-like folds, which produce the wavy lines of 
the internal surface. The two first layers are devel- 
oped from the free pallial margin, the last one from 
the epithelium of the mantle. The shell of the 
Gasteropoda and Cephalopoda consists largely of the 
porcellan layer, structurally different from that of the 
Ivamellabranchiata. Comparing the growth of the shell 
with that of the exoskeleton of the Arthropoda we 
find that the former leaves the animal free and enlarges 
with it through the addition of new layers, or through 
anterior enlargement, into which the growing animal 
moves, leaving the posterior smaller chambers vacant 
(Nautilus), whilst the latter must be repeatedly shed in 
order to allow the animal to grow. An originally asym- 
metric shell may in the course of time become symmetric 
or vice versa. The shell serves functions of protection, 
and therefore becomes complicated and variously coiled 
in freely moving animals which show the tendency to 
retract the whole body within their own shell. It has 
been definitely proved that forms with rudimentary or 
no shell at all must be derived from forms with a well 
developed shell. In all defective cases a shell generally 
becomes first internal, then decreases in size, the visceral 
sac becomes reduced, the shell occurs only in form of 
isolated calcareous corpuscles in the dorsal integument, 
finally even these disappear and the shell is only present 
in the embryo. The environments frequently require 
the reduction of the shell, but other compensations arise 
t o take its place. The valves of the Iyamellibranchia are 



MOI/LUSCA. 243 

connected by the hinge and the hinge ligament, consisting 
of an external non-elastic and an internal elastic layer. 
Many modifications occur. The pallial line, formed 
along the margins of the valves by the muscle fibres 
fastening the pallial margins forms in molluscs, provided 
with siphons, a characteristic modification, the ??iantle 
sinus, corresponding to the impressions of the sipho- 
retractor muscles and important for classification. 

The most important organ of the mantle cavity is the 
bra?ichial organ or gill, because mantle and mantle cavity 
were formed for its protection. It is a homologous organ 
derived from the gill of a common ancestral type, and 
therefore a definite morphological factor, designated by 
the name ctenidium in contradistinction from the secon- 
dary gills, which are morphologically different. The 
ctenidia of the Mollusca are originally paired, symmetri- 
cally arranged, pectinate, ciliary processes of the body 
wall, projecting from the trunk into the mantle cavity. 
Afferent vessels (branchial arteries) conduct venous 
blood into the gills, whence afferent vessels conduct the 
newly oxidized (arterial) blood back into the body, 
first into the heart. At the base of each ctenidium lies the 
osphradium (Spengel's organ). The gills have retained 
their most typical character in the Chitonidse, the Lamelli- 
branchiata, Cephalopoda and Zeugobranchia, only that 
in the latter the gills changed their original position. 
The asymmetry of the body manifests itself also in the 
gills, wherever onlyTme is retained. The ctenidia of the 
Testibranchiata (exceptions) have disappeared together 
with the mantle cavity, but may be replaced b}^ analogous 
adaptive gills. Since blood is carried from the gills into 
the auricles of the heart, there is an important relation 
between the two organs, i. e., the number and position 



244 MOI^USCA. 

of the gills corresponds to those of the auricles (except 
in the Chitonidae). In the Scaphopoda respiration is 
carried on by the soft portions of the integument. The chief 
forms of the adaptive gills of the Nudibranchia are (i) the 
anal gills of the Dorididse, (2) the right and left longi- 
tudinal rows of the subpallial gill plates of the so-called 
Phyllidiidse, (3) the dorsal appendages or cerata of the 
Nudibranchiata and of most Ascoglossa. In the Pulmo- 
nata the mantle cavity becomes a lung cavity. On the 
internal delicate surface of the mantle a dense respiratory 
blood- vascular network arises. A vein, the circular vein, 
runs along the mantle thickening, sending out numerous 
fine, anastomosing vessels, which enter into the large 
lung vein (arterial blood), parallel with the rectum, and 
thence into the auricle. Similar cases occur in the ter- 
restrial Prosobranchia. 

The hypo branchial gland is situated at the base of the 
ctenidium or betwen it and the rectum. It assumes 
various forms, but never becomes follicular or tubular. 
The purple gland of the Prosobranchia is such a gland. 

The absence of the head in the Lamellibranchiata is 
due to the peculiar development of the mantle and shell 
which separate the anterior region from the external 
world, and make the existence of a special highly sensi- 
tive cephalic region superfluous. Specially noteworthy 
are the cephalic foot of the Cephalopoda, the head of the 
(Gasteropoda and the oval anterior ljjad-like prolongation 
of the Scaphopoda. 

The mouth opening of the Lamelliabranchia continu.es 
to the right and the left in a furrow which extends pos- 
teriorly to the base of the gills. Its function is to con- 
duct food particles to the mouth. It is bordered by septa 
which form a kind of upper and lower lips; they may hang 



MOIXUSCA. 245 

into the mantle cavity in form of thin triangular plates, 
called oral lobes or palpi. 

The strong muscular foot is evidently the remnant of 
the ventral portion of an original cutaneous muscular 
envelope, adapted to crawling, whilst the formation of a 
shell made the dorsal portion superfluous and caused its 
reduction. The foot with a flat sole is no doubt the or- 
iginal type. It may be greatly modified by adaptation. 
An anterior portion (propodium) may become distinct 
from a posterior position (metapodiuni) which bears the 
operculum wherever it occurs; or parapodia may arise by 
lobe like enlargements of the margin of the sole; or an 
epipodium, a projecting fold, may develop around the 
base, i. e., the upper part of the foot; it is here where 
tentacular processes occur. In the Heteropoda the 
(medio- ventral) propodium has adapted itself to the 
function of swimming and transformed into a vertical fin, 
the parapodia of the Tectibranchia have changed into 
paired fins ox wings. In the Cephalopoda the foot has 
gradually moved towards the head, finally fusing with 
it, so that the arms (brachial umbrella) are considered 
lateral processes of the foot, especially on the ground 
that the sub oesophageal branchial ganglion which inner- 
vates the brachia, represents an anterior differentiation 
of the pedal ganglion, further because the brachia arise 
ontogenetically behind the mouth on the ventral side; 
their later position is only a secondary one. It can 
hardly be doubted that the two lobes of the funnel are 
epipodial lobes. The chief pedal gla?ids of'the Gastero- 
poda consist of an anterior glands (salivary) situated in 
the Prosobranchia between the two pedal lips (lobial 
gland), in the Pulmonata in the median basal line form- 
ing an epithelial canal; and of the unpaired sole gland, 



246 MOLIyUSCA. 

occurring largely in theProsobranchia, where they secrete 
a saliva that can be drawn out into threads, by means of 
which the animals attach themselves to foreign objects; 
it is homologous with the byssus gland of the L,amelli- 
branchia. The foot, the latter is generally laterally 
compressed, originally ending in a disc with marginal 
indentations. The byssus apparatus consists of (1) the 
byssus cavity, (2) the eductory canal, (3) the byssus fur- 
row, extending from the opening of the canal to the an- 
terior point of the foot, and (4) of a cup-shaped or semi- 
lunar enlargement of the furrow. The animal fastens 
itself by means of the byssus threads to foreign objects. 
The primitive internal orga?iization of the Mollusca is 
as follows: The alimentary canal. The mouth leads into 
a muscular pharynx with horny jaws; on its floor lies a 
hard plate called tongue or radula, which in successive 
transverse rows bears sharp, chitinous teeth. Paired sali- 
vary glands open into the pharynx. This is followed by 
an oesophagus leading into a midgut, which assumes a 
coiled shape and traverses, surrounded by large digestive 
glands, the body, ending posteriorly by means of a very 
short hindgut in a median anus. Wherever the body has 
developed a dorsal visceral sac (Gasteropoda) the ali- 
mentary canal forms a dorsal loop. Secondary loops or 
coils of the midgut are especially prominent in herbiver- 
ous Mollusca. The digestive gland, or so-called liver, 
unites the functions of the different specialized digestive 
glands of the Vertebrata. The radula is not present in the 
Lamellibranchia; they are, therefore, called Aglossa in 
contradistinction from all other Mollusca the Glossophora; 
the hard jaw and salivary glands of the latter are like- 
wise wanting in the former. The reason of this distinc- 
tion in the L,amellibranchia is to be sought in the fact 



•MOU,USCA. 247 

that they feed after the manner of sessile animals ; 
microscopic organisms are carried from the incnrrent 
water by ciliary motion into the mouth; there is no need 
of special masticatory organs. The anal gland of some 
Gasteropoda and the so-called ink sac of the Cephalopoda 
are connected with the terminal part of the hindgut. The 
wall of the alimentary canal is attached by connective 
fibres or bands, and consists of an internal, usually cili- 
ary, epithelium, an external muscular layer and an ex- 
ternal connective envelope. Pharynx and perhaps also 
parts of the oesophagus arise ontogenetically from the ec- 
todermal stomodaeum or proctodeum. The tongue is a 
tough, muscular, longitudinal structure, covered with a 
strong cuticular basal membrane, upon which numerous, 
often thousands of chitinous little teeth arise, arranged in 
transverse and longitudinal rows. Basal membrane and 
teeth together constitute the radtila, which is of the 
highest systematic value. It has its origin at the base of a 
sheath, into which it penetrates. Pharynx and tongue ap- 
paratus are protrusible, forming a very long proboscis in 
some predatory Prosobranchia. The oesophagus is situated 
between pharynx (wanting in L,amellibranchia) and stom- 
ach; there is hardly a line of demarkation. The stomach 
is that part of the midgut into which the liver opens. The 
liver is either a tubulous or acinous gland, representing a 
compact multilobed organ of brownish color. It consists 
of hepatic, ferment and calcareous cells. One of the di- 
verticula of many Lamellibranchia contains a rod-shaped, 
gelatinous cuticular mass, the crystalline style, whose 
nature is still disputed. Numerous modifications of the 
midgut occur, especially in the Cephalopoda (pancreas, 
spiral column)/ In the Aphistobranchia chitinous forma- 
tions occur in the stomach. The tubular intestiyie of the 



248 MOU,USCA. . 

midgut exhibits numerous coils and loops. In the large 
majority of the Lamellibranchia and Diotocardia the 
rectum perforates the heart, which suggests a relation- 
ship of groups. 

The primitive muscular structure consists of the mus- 
cles of the foot, of the muscles extending from the shell 
to the foot and the head {muscle of the spine, shell muscle), 
and of the muscles of the individual organs. The structure 
of the muscles is not cross- shifted. The spindle muscle 
{musculus columellaris) is attached to the spine in the 
interior of the shell, extends along the visceral sac and 
enters the dorsal side of the foot, where it radiates. It 
constitutes the specific retractory muscle, and is as such 
most highly developed in the Pulmonata where it forms 
the retractory muscles of the tentacles, eye-stalks and 
buccal mass and the muscles extending to the viscera. 
In the Lamellibranchia we distinguish the pallial muscles 
and the muscles terminating in the foot. The former is 
especially developed towards the free edge of the mantle 
and consists of three systems (1) the perpendicular mus- 
cular fbres of the pallial margins, (2) the muscle fibres 
running parallel to the pallial margin, (3) short fibres ex- 
tending from the internal to the external pallial surface. 
Differentiations of the pallial muscles are the siphonal 
retractors and the adductor muscles. Mollusca with two 
equal adductory muscles are called Dimyaria, those in 
which these muscles are unequal, Heteromyaria, and 
those with only one adductor, Monomyaria. The muscle 
.pairs extending from the internal surface of the shell 
into the foot correspond to the columellar muscles of the 
other Mollusca. In the Cephalopoda a cartilaginous 
entoskeleton is developed which serves, on the one hand, 
purposes of attachment for various muscles, muscle 



MOIXUSCA. 249 

groups and membranes, on the other hand purposes of 
protection for important organs, especially fois the central 
parts of the nervous system and the eyes. Constant is 
only the cephalic cartilage. Muscles are attached to the 
cartilage of the shell (depressor infundibuli ; retractor 
capitis lateralis; capitis medianus), or of the fin, or of 
the head (musculus collaris: adductor infundibuli). 

The primitive nervous system consists of two well- 
developed cerebral ganglia, situated on the dorsal side of 
the head and connected by a short cerebral commissure, 
and of two pairs of strong ga?iglio?iic nerve t) links, pro- 
ceeding each from a central ganglion and traversing the 
w T hole length of the body. The two trunks of the one 
pair, the pedal cords, extend along the right and left side 
of the foot, the visceral cords occupy a deeper and more 
dorsal position, extending along the body cavity and 
connected posteriorly. Excepting the Amphineura and 
the Diotocardia, the nervous system is modified as 
follows : two cerebral ganglia, two pedal ganglia, two 
pleural ganglia (on both sides of pharynx), two visceral 
ga?iglia (in the posterior region of the body cavity). 
Naming the ganglia of one and the same side connectives 
and the nerves, connecting the paired ganglia of both 
sides, commissures, we distinguish (1) cerebral commis- 
sures (above the foregut), (2) pedal commissures (below 
foregut), (3) visceral commissures (below rectum); fur- 
ther (1) cerebro-pedal connectives, (1) cerebro-pleural 
connectives, (3) pleuro-pedal connectives, (4) pleuro- 
visceral connectives. The nervous system of the 
Amphineura corresponds in the main to the original 
primitive nervous system of the Mollusca. The pedal 
and visceral cords are connected anteriorly by a gan- 
glionic cerebral semi-chcular co?d, and exhibit through- 
1* 



250 MOLLUSC A. 

out the body a great resemblence to the rope-ladder 
nervous system of the Turbellaria and Trematoda. 
In the Solenogastres the tendency arises to form localized 
ganglion centres, with a fused cerebral ganglion, three 
pairs of posterior visceral ganglia and two anterior pedal 
ganglia. The typical nervous system of the Gasteropoda 
is the modified primitive system given above, but it be- 
comes highly interesting through the formation of cruci- 
form plem o-visceral connectives, occurring in the Proso- 
branchia. The connectives cross each other in such a 
way that the one starting from the right pleural gang- 
lion extends above the alimentary canal to the left side 
before reaching the visceral ganglion, whilst the con- 
nective proceding from the left pleural ganglion extends 
beneath the alimentary canal towards the right side. 
Thus the parietal ganglion (in the pleuro-visceral con- 
nective) of the right pleural connective becomes a supra- 
i?itestinal ga?iglion (on the left side): the one from the 
opposite direction becomes a subintestinal ganglion (on 
the right side). The Prosobranchia are therefore strepto- 
neurous Gasterapoda, whilst the Opisthobranchiata and 
Pulmonata are euthyneurous Gasteropoda. The cerebral 
ganglia innervate the eyes, auditory organs, tentacles, 
proboscis, lips, retractory muscles of proboscis and buccal 
mass; the pedal ganglia furnish nerves for the pedal 
muscles (rarely for the spindle muscles); the pleural 
ganglia innervate mantle, spindle muscle and post- cepha- 
lic body wall ; the parietal ganglia, the ctenidia, osphra- 
dium and partly the mantle; the visceral ganglia, the 
visceral sac. The connectives and commissure may also 
furnish nerves for the neighboring ganglia. The buccal 
ganglia innervate the pharynx, salivary glands, oesopha- 
gus, anterior aorta, etc, The nervous system of the 



MOU.USCA. 251 

Seaphopoda is symmetrical, the visceral connective not 
cruciform, no separate parietal ganglia ; the main ganglia 
lie closely together. The nervous system of the Lamel- 
lobranchiata consists of the cerebro- pleural ganglia (fused) 
with commissures, the pedal ganglia and the viscero- 
parietal ganglia with commissures, all three connected 
by long connectives. The symmetric nervous system of 
all Cephalopoda is characterized by the marked concentra- 
tion of all the ganglia, especially in the Dibranchiata, 
where the central nervous system is enclosed by the ce- 
phalic cartilage. The buccal ganglia represent the sy?n- 
pathetic nervous system. 

The integument of the Mollusca contains a large num- 
ber of variously arranged epithelial sensory cells. They 
are of two kinds, either large surface cells (only in I^a- 
mellibranchiata), carrying tufts of externally projecting 
sensory hairs (brush- cells), or elongated filiform or spin- 
dle cells, which show a slight elevation near the nucleus. 
Both kinds of cells continue at their base in a nerve 
fibre, leading to the nervous system. They have no 
specialized functions, unless they combine into special 
sensory orga?is. Between these sensory cells other epi- 
thelial cells occur, e. g., glandular, ciliary, supporting 
cells, etc. The function of touch manifests itself wher- 
ever the integument is exposed. Sensory cells occur 
also on the internal epithelium of the mantle; they may 
be of an indifferent, or of a glandular or of a sensory 
nature. A whole region may assume their character 
wherever the one or the other kind predominates. The 
osphradium of the Prosobranchia is such an organ, per- 
forming ttit function of smell. Tentacles of smell occur 
in terrestrial Pulmonata and certain Opisthobranchia, 
whilst in the Dibranchiata distinct imiervated pits may 



252 MOLIvUSCA. 

take their place; similar organs occur on both sides of 
the anus in certain Asiphoniata. Here belong likewise 
the lateral organs of Diotocardia, the sensory organs of 
the Chitons (subradial, megalaestheta and mikraestheta), 
and the oral organs of taste in many Mollusca. All 
Mollusca, except the Amphineura have auditory vesicles 
(otocysfs), whose epithelial wall consists of ciliated and 
sensory cells; the vesicle contains otoliths of various 
numbers (i-ioo), size and composition They are gener- 
ally situated upon the pedal ganglia or near them. They 
reach their highest development in the Heteropoda and 
the Cephalopoda, where maculae and Cristas acusticae 
are formed. Byes occur in the form of cup-shaped (in 
Diotocardia and Nautilus), and vesicular eyes, (see p. 
56-58). The eyes of the Dibranchiata belong to the 
highest in the animal kingdom (p. 56). The dorsal 
eyes of Onchidium and of Pecten and Spondylus belong 
to the vesicular eyes, but here the external wall develops 
into the retina whilst the internal wall represents pig- 
mented epithelium. The compound or facetted eyes of 
Area and Pectunculus are epithelial organs correspond- 
ing to certain simple Arthropodal eyes. Originally the 
eyes of the Mollusca were evidently homologous, but 
adaptation brought about a change. 

The primitive original blood- vascular system is partly 
lacunary. The hea?t is arterial and situated in the peri- 
cardium above the rectum. It consists of a ventricle and 
two lateral auricles. In symmetric Mollusca with high 
visceral sac, it is displaced behind the rectum In the 
asymmetric Gasteropoda the position of the heart de- 
pends upon that of the pallial complex. Usually two 
large atteries (aorta) arise (frequently with one stem) 
from the heart, the one proceeding to the head, the 



MOLLUSC A. 253 

other to the visceral sac whence they anastomose and 
empty into the lacunae. The blood circulation has been 
described before. The blood ox, better, the hcemolymph, is 
rich in albumen (haemocyanin), and contains free swim- 
ming amcebid cells, which either separate from localized 
blood glands or from larger vascular regions. The wall 
of the heart consists of densely packed, smooth muscle 
fibres and of an external entothelium (pericardial). An 
internal entothelium is wanting. The wall of the ventricle 
is always stronger than that of the auricles. Valves occur 
between the two (atrioventricular) as well as in the pe- 
ripheral canals. The heart ma}' also be innervated. The 
somewhat complicated blood-vascular system of the Ce- 
phalopoda deserves special attention. 

We distinguish in the Mollusca a primary and a secon- 
dary cavity. The former represents the lacunar and 
si?ius system of the body, into which the arteries open and 
from which the veins, whenever they are present, receive 
their blood. It is without its own epithelial wall. The 
so-called secondary body cavity or the ccelum is in the 
large majority of Mollusca greatly reduced and consists 
usually only of the pericardium and the cavity of the 
gonads (testes, ovaria, hermaphroditic glands). It is 
always lined by its own epithelial wall, the entothelium 
of the blood cavity. It communicates externally through 
the nephridia (same as in Annulata). The germ layers 
are accumulations of the entothelium, so also the peri- 
cardial gla?ids. 

The nephridia {organ of Bojanus) consist typically of 
two symmetrical sacs which open externally through 
the two nephridial apertures into the mantle cavity, 
and through two internal apertures (infundibulum) into 
the pericardium. Their wall is richly saturated with 



254 MOIXUSCA. 

venous blood which here deposits its excretory sub- 
stances. Modifications and relations to the genital 
glands have been indicated before. 

The sexual organs consist of the gonads, the ducts and 
the organs of copulation. The gonads are either com- 
pletely or incompletely separated parts of the secondary 
body cavity; they are paired and symmetrical in the 
Lamellibranchia and Solenogastres. In all others they 
are single. Two pairs of gonads may be present in 
Hermaphrodites, but ova and spermatoza are here usually 
produced by one and the same gonad. (See classifica- 
tion.) They may be situated above the alimentary 
canal (Solenogastors) or in the visceral sac (Gastropoda, 
Scaphopoda, Cephalopoda) or typically in the primary 
body cavity above the muscular part of the foot between 
the alimentary coils. The reproductive cells may arise 
everywhere from the epithelium of the gonads or only 
in certain definite regions; they mature, separate and 
enter the secondary body cavity whence they are carried 
out. The gonads empty the mature eggs either into 
special ducts or into the nephridia. In many molluscs 
they pass through the pericardium (Solenogastres) before 
reaching* the nephridia. Whenever the gonads open 
directly into the latter, they either enter its proximal 
(communicating with the pericardium) or its distal por- 
tion (urinary duct) or a urogenital cloaca. The relation 
between paired ducts and paired or unpaired gonads is 
indicated in the classification. Whenever special ducts 
are present accessory pouches or glands or organs of copu- 
lations arise which are most complicated in hermaphro- 
ditic Pulmonata and Opisthobranchia. In the Gastero- 
poda a male organ (either a freely projecting penis or a 
similar protrusible organ) arises in the cervical region. 



MOLLUSCA. 255 

In the Cephalopoda a modified (hectocotylized) arm of 
the male assumes the function of copulation. The sper- 
matazoon is usually pin- or hair- shaped. In parasitic and 
sessile snails the organization becomes rudimentary. 
Certain Gasteropoda are viviparous. 

Ontogenetic and phylogenetic characteristics. The Mol- 
luscan egg generally undergoes a total unequal segmen- 
tation (discoidal in Cephalopoda), resulting either in an 
invagination or an epibolic gastrula (p. p. 38-40). The 
characteristic larva is the veliger- larva, i. e., a trocho- 
phora with Molluscan characteristics : (1) the dorsal 
shell-gland with the embryonic shell (or shells), and (2) 
the ventral foot-rudiments. Metamerism occurs before 
all the trochophora characters have become fully devel- 
oped. The preoral ciliary circle (the velum) of the Mol- 
luscan larva corresponds to the same structure in the 
Turbellarian larva ; the primitive nephridium of the for- 
mer to a simplified water- vascular system of the latter, 
whilst the constant nephridia as genital ducts are 
homologous with the genital ducts of the Turbellaria. 
With the acquirement of rectum, heart and secondary 
body cavity the Mollusca rose above the modern 
Platoda, whilst they are still one (in the lower forms) 
with them with regard to the nervous system. The 
secretion of a shell changed the respiratory functions of 
the body surface and led to a formation of localized 
gills ; at the same time the muscular structure of the 
dorsal side, and with it the longitudinal nerve cords, 
became reduced, whilst the opposite took place with 
the ventral muscular structure, which developed into a 
foot; the dorso-ventral muscles, however, into a shell- 
muscle, 



ECHINODERMATA. 



The Echinoderms are exclusively marine animals. 
They exhibit externally a radial usually pentamerous 
arrangement. This radial symmetry is never perfect, 
the body is in reality bilatterally symmetrical in rela- 
tion to a median plane which traverses the centre of 
length of one of the radiating metameres. They are al- 
ways provided with a skeleton, composed of calcareous 
spicula, which usually unite into networks, giving rise to 
definite skeletal plates. An alimentary canal is present 
whose walls are separated from the body walls by a spa- 
cious peritoneal cavity. A system of ambulacral vessels, 
composed of a circular canal around the mouth, which 
sends off a branch canal with numerous lateral prolonga- 
tions along the middle line of each of the ambulacral 
metameres, represents the highly characteristic water- 
vascular system. Hatschek, therefore, names this group 
Ambulacralia, classifying them together with Enterop- 
neusta as a special type. A true vascular system is also 
present. The nervous system consists of a wing which 
surrounds the oesophagus and gives off radiating longi- 
tudinal cords. Reproduction is mainly sexual, jand sepa- 
rate sexes are the rule. The development presents a 
complicated metamorphosis, the embryo leaving the egg 
as a bilaterally symmetrical larva, which has different 
names in different classes. 

I CiyASs: Crinoidea (Feather- Stars). Bursiform or 
cup-shaped Echinodermata. Inferior or dorsal integu- 



CRINOIDEA. 257 

ment of the body (calyx) always provided with polygonal 
plates, articulated by their edges; in the ventral integu- 
ment sometimes only scattered calcifications. At the 
border of the calyx from two to eighteen segmented 
arms (brachia) arise, be.iring ambulacral appendages 
(pinnules), which are continuations of those situated 
along the ambulacral furrows of the calyx. Attached 
by the apical pole either directly or, more commonly, by 
means of a calcareous stem to submarine Objects. Seg- 
ments of the stalk usually pentagonal, connected by 
bands of tissue. It is attached at its distal end by a root- 
like expansion or by numerous branched cirri; at certain 
intervals it bears hollow and segmented cirri, which are 
arranged in whorls. Through the centre of the stem 
runs a canal containing a soft, solid substance, and per- 
forming nutritive functions. Mouth and anus situated 
on the ventral side of calyx. Generative rhachis in the 
arms. Largely extinct (Paleozoic). Represented by 
only a few living genera, 

1. Order: Tesselata. Calyx formed of calcareous plates, 
oval face without ambulacral furrows. Cyathocrinus. 
Actinocri?ius. Extinct. 

2. Order: Articulata. Oval face of calyx, usually 
membranous, or sub membranous, with ambulacral fur- 
rows. 1. Family: Pe7itacrinid<z . Always attached by 
a pentagonal stalk, with whorled cirri Pentacrinns caput 
Meduscz and P. Midleri, Rhizocrinus Holopus. Fossil 
forms: Encri?ius liliiformis. Apiocrinus, etc. 2. Family: 
ComatulidcB : Attached only in the young state. Usually 
ten arms at the margin of the flattened body by means 
of which they propel themselves; mouth and anus pres- 
ent. The vermiform larva swims about freely, and passes 
later into the stage of the stalked Pentacrinus, from 



258 ECHINODERMATA. 

which the Comatula is produced by the separation of the 
cup from the stalk (Claus). Antedon (Pentacrinus 
Europseus). Adinometra. 

I Appendix: Cystidea. With short stalks and slightly 
developed arms. Generative organs enclosed in the 
calyx, issuing through movable valves, arranged around 
a central pore. Sph&ronites . Caryocrinus . Apicystites. 
Extinct. 

2. Appendix: Ederioasterida. Fossil. Ambulacra per- 
forated by canals, opening directly into the calyx-cavity. 
Without anus and stalk. Edrioastet. Agelacrinites. 
Hemicy slides. 

3. Appendix: Blastoidea. Prismatic body. Without 
arms ; calyx with ambulacral areas, attached by a short 
jointed stem. Pentatrematites . Extinct. 

II Class: Echinoidea. Body spheroidal, oval, dis- 
coid or heart-shaped, with immovable skeleton (shell, 
test or perisome), composed of calcareous pentagonal 
plates, bearing movable spines and usually arranged in 
twenty meridional rows, which correspond in alternate 
pairs with the radii and interradii (corona). The radial 
pairs, representing the ambulacral plates, emit the am- 
bulacral tube feet through fine pores arranged in rows. 
Both ambulacral and interambulacral plates bear spheri- 
cal prominences and tubercles to which the differently 
shaped movable spines are joined. Between the latter 
pedicellariae occur, which are especially numerous in 
the region of the mouth. The genital pores are situated 
on the interradial genital plates near the apical pole. 
Beneath them lie the reproductive organs, large racemose 
glands. One of these plates is perforated by numerous 
fine holes, serving as a communication between the stone 
canal and the outside (madreporic plate). The arrange- 



ECHINOIDEA. 259 

ment of the nerves and the ambulacral vascular trunks 
is highly characteristic of the internal organization. 
There is invariably a mouth surrounded by oval plates, 
the peristome, (frequently provided with teeth) and anus 
protected by anal plates; the periproct and the ambula- 
cral appendages have often respiratory functions. Echi- 
noids live near rocky coasts. Many are fossil. 

1. Section: Regularia or Entocyclica. Mouth central or 
sub-central ; anus sub central in the apical space. 

1. Order: Cidaridea. Sea-urchins. With teeth and 
masticatory apparatus, with equal band -shaped ambu- 
lacra. Madreporic plate in the anterior right inter- 
radius near the apex. 

A. Shell round. Families: Cidaridcz, with smooth, per- 
forated tubercles; ten anal plates; no buccal branchiae. 
Cidaris. Diatematidcz, with crenulate perforated tuber- 
cles, peristome notched. Arbaciad<z, with smooth imper- 
forate tubercles; four large anal plates; auriculae closed. 
Arbacia. Echinid<z, imperforate tubercles or perforate 
and crenulate; numerous anal plates; pairs of pores in 
rows of three, four or more. Strongylocentrotus. EcJmius. 
Hernias ter. 

B. Shell oval or elliptical. Echinometridcs . Pores in 
rows of five or six pairs. Echi?icmetra. Podophora. Acro- 
cladia. In the family SalenidcE the anus has become ec- 
centric through one or more supernumerary apical plates. 
The only living form: Salenia rarispina. Echinothuridce 
extinct. 

2. Section: Irregularia or Exocylica. Anus eccentric, 
not within the apical disc. 

2. Order: Clypeastridea. Irregular Echinoidea, more or 
less shield-shaped. Mouth usually central, provided with 
masticatory apparatus; five -leaved ambulacral rosette 



260 ECHINODERMATA. 

round the apical pole; very small tube feet; no Polian 
vesicles. Five genital pores in the region (apical) of the 
madreporic plate. Ambulacra less petaloid. Families: 
Clypeastridce . Shell flattened, disc without indentations, 
subpentagonal. Clypeaster. Echinocyamus. Scutellidtz '., 
shell flattened, often lobed or perforated, lower surface 
with ramifying grooves. Lobophora. Rotula. 

3. Order: Spatangidea. Heart-shaped; dental appa- 
ratus absent; mouth and anus usually eccentric; four- 
leaved ambulacral rosette and four genital plates; without 
Polian vesicles. Madeporic plate interradiate. Families: 
Spatangidce, mouth transverse or reniform. Spatangus. 
Sch iz aster. A mph idotus . 

Appendix: Fossil forms with simple ambulacra : Gal- 
eritidcB, mouth central, shell globular or subpentago- 
nal, a single apex. Dysasteridce, mouth eccentric, shell 
ovoid, two apices, towards which the bivium and trivium 
converge. 

Ill CiyASS: Ophiuroidea. The brittle stars have a 
general external resemblance to the Asteroidea. The 
ambulacra are confined to the oral side of the body ; the 
ambulacral grooves are covered by a series of dermal 
plates, the feet projecting at the sides of the arms, be- 
tween the spicules on the upper surface. The arms aie 
flexible and cylindrical, sharply distinct from the disc ; 
they are rarely branched and can (by means of longitudi- 
nal muscles) be rolled up in the direction of the mouth. 
The latter is situated in the centre of the oral surface, 
surrounded by five oral angles, each consisting of five 
pieces. The alimentary canal is a simple gastric sac 
without cceca and has no intestine or anus; pedicellariae 
are likewise wanting, the madriporic body situated in 
one of the buccal plates. The genital glands are lodged 



ASTEROIDS A. 26l 

in the disk, and pour their products into genital pouches, 
passing directly to the exterior through inter-radial 
paired slits. A few viviparous Ophiuroidea do not 
undergo metamorphosis. 

1. Order: Ophiuridea. Arms unbranched; ambulacral 
furrows covered with plates, genital clefts ordinarily 
five, habits creeping. A. Oral clefts aimed, no palae 
angulares (teeth-like processes): Obhiodermatidce \ disc 
granulated. Ophhira. OphiolepidcB, disc scales naked. 
Ophiolepis. Ophiopus. Amphiuridce, disc rugged and 
scaly. Amphiura, Ophiopholis. Ophiomyxidce, disc 
naked. Ophiomyxa. Oral clefts with palse angulares: 
Ophiocomidcc , disc covered with solid plates. Ophiocama, 
B. Oral clefts unarmed. OphiothricidcE, radial plates very 
large. Ophiothrix. 

2. Order: Euryalidea. Arms simple or branched, and 
capable of being rolled up towards the mouth, without 
plates. No spines, but tufts of papillae on the ventral 
surface of the arms; ten genital slits. Astrophytori. 
Asteronyx. 

IV Class : Asteroidea. (Star-fish.) Echinoderms 
with dorso-ventrically compressed pentagonal or star- 
shaped body; ambulacra restricted to the oral half. The 
skeleton is made up of plates or thick rods, composed of 
a dense calcareous network. A deep groove radiating 
from the mouth to the end of the ray maiks the 
position of each ambulacrum, laterally supported by 
two series of ambulacral ossicles connected by trans- 
verse muscles and articulated together like vertebrae in 
the middle line or roof of the grove, with spaces between 
their lateral processes for the passage of the vessel con- 
necting the ampullae with the radial vessel and the tube 
feet. The ambulacral nerve and canal are situated out- 



262 ECHINODERMATA. 

side the ossicles. At the circumference of the oval disc 
the ossicles of the ambulacra, smaller, and closely united 
together form a pentagon, the angles of which correspond 
to the ambulacral grooves, round the oesophagus. The 
joined outer ends of the pair of ambulacral ossicles near- 
est the mouth project on the oval side and are armed 
with strong spines, covered with pedicellariae, small, 
pincer-like bodies, which during life twist and snap 
about. The wall of the body upon the aboral face and 
the sides of the rays is also armed with spines, to which 
pedicellariae are attached. Mouth always in the centre 
of the ventral surface, in a pentagonal or star- shaped 
depression, surrounded by stiff papillae. Five pairs of 
Polian vesicles project from the circumoral canal; the 
madreporic canal branches off from the latter opposite 
one of the inter-radial folds; taking a sinuous course, it 
passes to the aboral surface, terminating beneath the 
madreporic body. Lines drawn from the mouth along 
each ambulacrum are termed radii; the regions between 
the ambulacra are inter-radial. Anus usually present at 
the aboral pole. Madreporic plate and genital pores 
inter-radially upon the aboral surface. Multi-lobed, 
branched diverticula of the stomach extend into the 
cavities of the anus, likewise the generative organs. 
Some are developed within the brood pouches of the 
mother; the majority pass through free larval stages. 
They all possess great powers of regeneration. Live on 
rocky coasts. 

i. Order: Asteriidea. Ambulacral feet end in broad 
suctorial discs, arranged in four rows. Asterias. Helias- 
ter. Anastetias. 

2. Order: Echinasteridea. Two rows of cylindrical am- 
bulacral feet. Families: Solasteridcs . Dorsal skeleton 



HOLOTHUROIDEA. 263 

reticulate. Acanthaster. Solaster. Echinaster. Linckiadcz: 
dorsal skeleton of longitudinally arranged, or rounded, 
or quadrangular vesicles; integument granulated. Ophi- 
diaster. Linckia. Goniasteridce v dorsal and ventral mar- 
ginal plates very distinct. Pentagonaster. Goniodiscus. 
AsierinidcE: skeletal vesicles imbricated. Asteri?ia. Pal- 
mipes. 

3. Order: Astropirtinidea, ambulacral feet conical and 
without suctorial disc. Families: Astropedinidce, skele- 
ton of paxillae. Astropecten. Luidia. Cteriodiscus. 
Pte7-asterid<z . Integument supported by spines, radiat- 
ing from the prominent skeletal ossicles. Pteraster. 

4. Order: Brisingidea. Body shaped like an Ophiu- 
roid. Rays long, distinct from disc with only a narrow 
internal cavity. Brisinga. 

V Class: Holothuroidea. (Sea cucumbers.) Ver- 
miform Echinoderms. The body-wall consists of an 
external cellular ectoderm, covering a layer of connec- 
tive tissue, within which are circular and longitudinal 
muscular fibres, arranged in five bands and attached 
anteriorly to a corresponding number of the pieces of a 
calcareous oesophageal ring (composed of 10 or 12 sepa- 
rate ossicles). The Holothurians exhibit bilateral sym- 
metry and resemble greatly the Gephyrea. The ambula- 
cral feet may be either confined to the three rays of the 
trivium, or uniformly distributed over the surface of the 
body (especially the ventral side) or wanting altogether. 
Contractile tentacles surround the mouth, communicat- 
ing with the water vascular system and representing 
specially modified ambulacra. The stone canal hangs 
freely in the body cavity, ending in a calcareous appara- 
tus the madreporic plate. Respiratory trees and excre- 
tory glands (Cuvier) may be present at the end of the 



264 ECHINODERMATA. 

intestine. The generative organs consist of branched 
ccecal tubuli, whose duct opens dorsally near the mouth. 
They are of separate sexes, except Synapta. Develop- 
ment usually direct. The Holothurians bury themselves 
in the sand near the coast. They likewise possess great 
powers of regeneration'. 

1. Order: Apneumo?ia. Respiratory trees and Cu- 
vierian organ wanting; mouth and anus at opposite ends 
of the body; five ambulacral furrows; hermaphrodites. 
Families : Synaptidce. Without pedicels. Synapta. 
Chirodota. Onci?w la bides . Without pedicels. Echino- 
soma. 

2. Order: Tetrapneumona. Four respiratory trees; 
body flask- shaped ; mouth and anus at the same end; 
the former surrounded by ten tentacles and ten calcareous 
plates, the latter by as many papillae and plates. Only 
genus : Rhopalodina lageniformis. 

3. Order: Dipneumona. Two respiratory trees; Cu- 
vierian organs present ; mouth and anus polar ; pedicels 
in a single row. Families: LiodermatidcE . No pedicels; 
tentacles varying. Liosoma. Dendrochirotce, tentacles 
branched. Thyone. Cucumaria. Psoitis. Aspidochirotce, 
tentacles shield- shaped. Holothuria. 

Orgaiiization of Echinodermata . The primitive ances- 
tral type of the Echinodermata is most probably repre- 
sented by the stalked Cystidea, for it must be assumed 
that the influence of the sessile mode of life produced, 
just as in other groups, the radial structure. Their form 
is spheroid and in the simplest cases without brachial 
elongations. The protecting polygonal plates exhibit at 
first an irregular arrangement without any indication of 
radition. However, five radial furrows extend from the 
mouth, resembling the ambulacral grooves upon the disc 



SCHINODKRMATA. 265 

of a Crinoid or Asteroid. A continuation of the sessile 
mode of life largely aided in completing the radial struc- 
ture, until a type was reached similar to the following 
fundamental form. The body represents a sphenoid with 
the principal (long) axis somewhat shortened and the 
poles flattened and dissimilar; the one (oval) containing 
the mouth, the other (anal) the anus. Five planes pass- 
ing through the long axis of this spheroid will divide 
the body each into two bilaterally symmetrical halves. 
Thus the meridians arise which are separated from one 
another by equal intervals. Five alternate ones, called 
the principal rays or radii, contain the most important 
organs (nerves, vascular canals, ambulacral feet, etc.), 
while the other five meridians constitute the intermediate 
rays or inter-radii, also containing certain organs. But 
only the regular Echinoderms present a pentamerous radial 
arrangement with complete radial and interradial sym- 
metry; however, since certain organs always remain 
unpaired; symmetry is, even here, not perfect. When 
one of the rays differs in size from the others an 
irregularity arises, the plane of the unpaired ray forming 
a median plane, on each side of which two pairs of equal 
rays are repeated. These form the right and left sides 
of the body, whilst the unpaired radius constitutes the 
anterior, the paired radius the posterior end; the apical 
pole represents the upper surface, the oral pole the under 
surface. In the irregular Sea-urchins this symmetry 
becomes still more perfect, inasmuch as the anus occurs 
on the ventral side of the unpaired inter-radius Clypea. 
stridea), or both poles (sometimes only the oral) are 
shifted in the direction of the unpaired radius, thus 
becoming eccentric {Spatangidce) . The distribution of 
the organs of locomotion {ambulacra) along all the five rays 

M 



266 KCHINODBRMATA. 

is of rare occurrence (in only a few regular Echinoder- 
mata). The oral pole usually becomes the ventral sur- 
face to which the organs of locomotion are confined. 
Irregular Echinodermata move in the direction of the 
unpaired ray, which causes the shifting of the oval pole 
in that direction, leaving the two posterior (bivium) radii 
to form the ventral surface. In the case of the Holo- 
thurians which obtained a cylindrical form by an elon- 
gation of the axis, the unpaired radius with its inter- 
radius marks the dorsal and ventral surfafce. The body 
is compressed in such a manner that three radii {trivium) 
with their organs of locomotion are placed on the ventral 
surface. In other cases the radii may become two or 
three times as long as the inter-radii until the body 
assumes the shape of a star which may be either flat or 
arched. (Compare classification.) 

The cellular ectoderm of the body of the Echinoderms 
is provided with a thin cuticle, bearing numerous cilia. 
Beneath this lies a mesoderm containing connective and 
muscular elements, in which calcareous deposits are lodged. 
These give rise to a solid, more or less movable skeleton, 
which constitutes an important charade* istic ot this group. 
A ciliated epithelium lines the perivisceral cavity. The 
separate elements of the skeleton are the ossicula, the 
spines attached to them by ligamentous fibres, and the 
calcareous structures contained in the pedicellaricE . On 
the antambulacral wall (of the Asteroidea) the ossicula 
(or plates) are generally elongated rods of very unequal 
lengths, with open meshes between and bearing different 
processes. Along the margin there is usually a row of 
larger calcareous plates, the superior marginal plates. 
On the oral surface we distinguish besides the internally 
placed ambulacral ossicles, the infeiior marginal vesicles, 



ECHINODERMATA. 267 

the adambulacral plates, and the inter- ambulacral plates ; 
the two last correspond to the inter-ambulacral plates of 
the Echinoidea (compare classification), but are disposed 
along opposite sides of adjacent arms. The spines of 
the Sea-urchins are moved laterally by special muscles 
developed in a soft superficial integumental layer. As 
to pedicellarise compare the notes given under the special 
classes. Around the mouth of the Sea-urchin and on 
the dorsal surface of the Starfish are small transparent 
bodies, sph<zridia, which are probably sense organs. In 
the Spatangidse there are peculiar bands upon the upper 
surface, the fascioles or Semites, which bear bristles with 
cilia (cJavidtz). In the Holothuroidea the skeletal struc- 
tures are confined to isolated calcareous bodies, embedded 
in the integument and resembling latticed plates, wheels 
or anchors. The dermal muscular system consists here of 
five pairs of longitudinal muscle- bundles surrounded by 
a layer of circular muscular fibres which line the internal 
surface of the integument. 

Perhaps the chief characteristic of the Echinodermata 
is the possession of the peculiar water vascular system 
and of the distensible ambulacral feet connected with it. 
It consists of the circumoral ambulacral vessel, lying close 
to the ossicles to which the margins of the oral mem- 
brane are attached, and of five radial canals extending 
into the rays. Along opposite sides. of the radial canal 
numerous short branches (internally ciliated) pass from 
it, each opening between the ambulacral ossicles into the 
neck of a contractile muscular sac {ampulla), situated in 
the interior of the ray. The neck of each ampulla passes 
in opposite directions into one of the ambulacral feet 
(pedicels), which are the lateral appendages of the five 
or more radial trunks, thus affording a communication 



268 3CHIN0D3RMATA. 

between them and the circumoral canal. They control 
by means of a water current the contraction and disten- 
tion of these feet, enabling them to fasten themselves 
by means of their sucking discs, and to move (in most 
cases) the body in the direction of the radii. The dis- 
tribution of these feet, and the formation of an ambula- 
cral and antambulacral zone, has been indicated in the 
classification. The circumoral canal of the Holothurians 
gives off anterior branches to the tentacles round the 
mouth. As each enters its tentacle, it dilates and sends 
down a short ccecal prolongation on the outer side of the 
calcareous ring. There are four kinds of ambulacral 
feet in the Spatangidea. They are either simple loco- 
motive organs, with or without suctorial discs, tactile 
feet with papillose extremities, or gill-like, more or less 
pectinated appendages, arranged over rosette shaped 
areas. In the Clypeastridea the petaloid portions of the 
ambulacra possess both branchial and delicate locomotive 
(with suckers) pedicels. Very frequently a number of 
small eminences, consisting of coecal vesicles (Polian 
vesicles) are seated upon and open into the circumoral 
canal; likewise a number of racemose appendages whose 
function is still disputed. A stone canal (rarely more 
than one) proceeding from the circumoral vessel permits 
the communication between the sea-water and the fluid 
of the water vascular system. It receives its name from 
the calcereous deposits in its walls. May be suspended 
within the body and take up fluid through the pores in 
its walls, or it may end in a porous, calcareous plate 
{madreporic plate or body), which is inserted in the exter- 
nal cavity of the body. Its position varies considerably, 
as indicated in the classification. 

The alimentary canal of the Echinodermata consists of 



ECHINODERMATA. 269 

oesophagus, stomach and rectum. The mouth is often 
surrounded by projecting skeletal plates provided with 
spicules. In the Cidaridea and Clypeastridea a powerful 
masticatory apparatus is developed known by the name 
of "Aristotle's lantern." It consists of five hollow, 
wedge-shaped, calcareous pieces — the alveoli — each of 
which is composed of two halves united together in the 
middle line; each half again consists of a superior 
(epiphysis) and an inferior principal portion. From 
each alveolus projects a long pointed tooth, and strong 
muscular fibres (inter-alveolar) unite the alveoli into a 
one shaped apparatus; theepiphyses of each pair of alveoli 
are connected by long radial (articulated) pieces, the 
rotidce, each of which articulates internally with a slender 
arcuated rod which terminates above the rotula in a free 
bifurcated extremity, the 7'adius. The alveoli and teeth are 
interambulacral, the radii androtulse are ambulacral. Pro- 
tractor muscles arise from the interambulacral region of 
the oval edge and are inserted into the upper part of 
the alveoli ; slender oblique muscles of similar origin are 
inserted into the radii ; transverse muscles connect the 
radii together and retractor muscles from the ambulacral 
arches are inserted into the oval ends of the alveoli. 
The oesophagus of the Asteroidea opens into a wide 
stomach enlarging into five cardiac sacs whose walls are 
composed of many sacculi. Each cardiac sac may extend 
a short distance into the cavity of the corresponding 
ray. On the aboral side of these diverticula the alimen- 
tary canal abruptly narrows and then dilates again into 
a pentagonal pyloric sac whose angles divide and pass 
as parallel multilobed diverticula (liver) along either 
(aboral) side of each ray. Pairs of mesenteric folds fasten 
the alimentary canal to the body wall. The five divert^ 



270 ECHINODERMATA. 

cula of the short rectum which fall in the inter- radii are 
much shorter and perhaps perform the functions of neph- 
ridia. The narrow intestine of Comatula, Sea-urchins 
and Hothurians is much increased in length and either 
toiled or folded. The anus of all Echinoderms is usually 
placed at the centre of the apical pole, rarely in an in- 
ter-radius on the ventral side. 

The true vascular system of the Echinoderms consists of 
two ring-like vascular plexus, one around the oesophagus 
and one on the aboral surface; the former sends off 
branched radial vessels, the latter supplies the stomach 
and genital organs. Both rings are connected by a so- 
called heart (connected with the stone canal in the As- 
teroidea) composed of a plexus of contractile vessels. In 
the Holothurians only the oesophageal circular vessel 
with two trunks, giving off branches to the intestine, 
has been observed. A watery fluid containing nucleated 
cells which exhibit amoeboid movements fills not only 
the blood-vascular system, but also the vessels and the 
peritoneal cavity of the body and rays. The vascular 
rings are the result of the metamorphosis of saccular 
diverticula of the alimentary canal (coelom), which have 
encroached upon and largely diminished the primitive 
body cavity. 

The function of respi? r ation is largely performed by the 
ciliated surface of the organs suspended in the body 
cavity and of the external appendages, which keep the 
traversing sea water in constant motion. Special organs 
of respiration are the ambulaaal branchicz of the irregular 
Sea urchins, the dermal branchics (simple tubes) covering 
the whole surface of the Asteroidea, the five pairs of 
branched tubes surrounding the mouth of Echini, and 
especially the so-called respiratory bees of the Holothuri- 



ECHINODERMATA. 27 1 

ans, two large tree- like branched tubes, which open by 
a common stem into the cloaca. The functions of the 
respiratory tree of the Starfish is still a matter of dis- 
pute. 

The central nervous system of the Kchinodermata con- 
sists of band-like thickenings of the ectoderm, which in 
the Asteroidea are situated, one in each ray, beneath the 
epidermis of the ambulacral groove, external to the blood 
vessel and water- vascular canal, sending off numerous 
fibres to the muscles of the spines, the ambulacral feet, 
pedicellariae, etc. Followed to the base of the rays they 
reach the oral disc at the periphery of which they divide 
and, skirting the margin of the disc, join to form a sub- 
pentagonal ring (containing ganglion cells) round the 
mouth; in the opposite direction they end upon the eyes 
and their tentacles. The eye at the ventral termination of 
each ray represents a spherical protuberance, the conical 
surface of which is covered by a membrane which covers 
a number of conical simple eyes, each consisting of a red 
mass of pigment around a refractive body and a nervous 
apparatus. In^the Cidaridea they represent five tentacle- 
like prominences with nerve endings, situated on special 
plates {ocular plates) at the apical pole. The tentacle-like 
ambulacral foot at the end of each ray (in Asteroidea 
and Ophiuroidea) seems to be of tactile character, just as 
the tentacles of the Holothuroidea and the tactile feet of 
the Spatangidse. 

The number and position of the genital organs gen- 
erally correspond to the radial structure. The five-lobed 
ova or testes of the Kchinoidea are situated in the inter- 
radii immediately beneath the genital plates around the 
apical pole through which they open ; in the irregular 
Spatangidae there may be only two or three glands the 



272 ECHINODERMATA. 

generative organs of the posterior inter-radius being 
always absent ; in the Asteroidea the five pairs of genital 
glands have the same interradial position, extending 
sometimes into the rays, and opening through numerous 
pores on the interradial dorsal side. In the Ophiuroidea 
ten lobed generative glands are developed around the 
stomach (comp. classification); those of the Crinoidea 
are concealed in the arms and pinnules ; those of the 
Holothuroidea are reduced to one branched gland the 
duct of which opens on the dorsal side of the interior 
pole. The organs of the two sexes are in all cases 
extremely alike ; they only differ somewhat in color, the 
seminal fluid being white, the ova red or yellow. Sexual 
differences or dimorphism is very rare. Ova and sper- 
matazoa first unite in the sea water, after ejection from 
the parental body. Internal fertilization only occurs in 
several viviparous species of Amphiura and Phillophorus 
(Claus). 

The development of the different classes of the Echino- 
dermata shows their close relationship. It manifests 
itself in four stages: (i) in the formation of the primary 
germ layers and of the mesenchyma, as well as of the be- 
ginning of mouth and anus; (2) in the rise of enterocoelum 
and hydrocoelum; (3) in the development of the typical 
larval forms; and (4) in the transformation of the larva 
into the Echinoderm. Segmentation is always total and 
nearly equal resulting in a ciliated blastula. From this 
a typical invagination gastrula is developed ; mesen- 
chyma cells separate from the invaginated part (Ante- 
don, Astropecten, Synapta). In the Kchinoidea and 
Ophiuroidea the mesenchyma arises before gastru- 
lation, but likewise from the entodermal portion of 
the blastula. The mesenchyma has the same origin 



ECHINODERMATA. 273 

as the mesodermal structures, which separate from the 
ccelom as ccelom sacs. The next process is the constric- 
tion of the embryonic structures of body cavity and 
water-vascular system, which arise as diverticula of the 
ccelom. Ludwig calls them enteroccelom and hydro- 
ccelom; Selenka uses for both the name vaso-peritoneal 
vesicle. They separate into two enteroccela and the 
hydroccelom, the latter of which opens externally by a 
dorsal porus. The blastoporus becomes wherever it is 
retained, the anus. The mouth originates through the 
union of ccelom and ectoderm ; different modes of life, 
however, condition variations in the formation of mouth 
and anus. Although the external form of the different 
larvae varies greatly, there are nevertheless certain ex- 
ternal essential points of resemblance, among which the 
common ciliated band is most prominent. Thus the* 
larva of Antedon, which differs from all others in form, 
resembles the so-called pupa of the Holothuroidea intfie 
possession of five ciliated bands. Even the development 
of the various systems of organs shows marked coinci- 
dences, so, e. g., the water vascular system, the nervous 
system and especially the muscular structure. The rise 
and final union of the ambulacral and anti-ambulacral 
surfaces are of the very same character in the Brachio- 
laria (Asteroidea) and Pluteus (Ophiuroidea) larvae. 
There exists a certain similarity between the larvae of 
Crinoids and Holothurians with regard to the formation 
of tentacles at the base of an oral vestibulum produced 
by an ectodermal invagination. Nothing definite can 
be said in reference to the various calcareous skeletons. 
All Echinoderms possess a radial structure but their 
larvae are of a bilaterally symmetrical structure, both 
internally and externally. The transformation or meta- 



274 KCHINODERMATA. 

morphosis of the latter into the former is simplest in the 
Holothuroidea. The lobed processes of the larva (Auri- 
cularid) disappear, the ciliated band divides into separate 
pieces, which change their longitudinal into a transverse 
position. The margin of the larval body becomes even 
and the larva assumes a cylindrical shape ; the pieces of 
the ciliated bands unite laterally into five rings surround- 
ing the body like loops (pupa stage); four of these pieces 
unite into a ring around the mouth and gradually move 
into the interior of the larva, forming finally the oesopha- 
geal nerve-ring. After this the water-vascular system 
differentiates by secondary anterior and posterior evagi- 
nations of the water- vascular ring ; finally the ambula- 
cral feet make their appearance, being evaginations of 
the ambulacral radial vessel, and the tentacles protrude 
through the mouth opening, the animal begins to move 
about. 

It is difficult to show their phylogenetic relation- 
ship. Various theories have been advanced^ but none 
is satisfactory. It is most likely that palaeontology 
furnishes the best solution of the question. The relation 
between modern Kchinodermata and the Cystidea has 
been indicated above. However, no one can tell where 
the bilateral ancestors of the radial type may be found. 
The trochophora larva shows certain resemblances, but 
the difference in the distribution of the cilia and the ab- 
scence of a vertex plate make a definite decision very 
difficult. It is however firmly established that the devel- 
opment of the body cavity as it takes place in the Annu- 
lata and that of the Bchinodermata indicate the same 
origin, so that the mesoderm of the Annulataand that of 
the Kchinodermata must be considered as homologous. 
There are even indications which establish the relation 



ECIHNODERMATA. 275 

of the Echinodermata to segmented forms. (Korschelt 
and Heider.) 

Appendix: Enteropneusta. The isolated genus 
Balanoglossus is generally classified under the above 
head and forms, together with the Echinodermata, Hat- 
schek's type Ambulacralia. Prof. E. D. Cope and 
others assign it as superclass I (Hemichoi da) a position 
among the Vertebrata. As such it consists of but one 
class the Enteropneusta with but one order the Helmin- 
thophya, represented by one family the Balanoglos sides, 
including species of worm-like form which burrow in 
the soil of the sea coast below high tide. Balanoglossus 
clavigerus (Delia Chiage). B. minutus (Kowalewsky). 
B. Kowalewskii (Al. Agassiz). B. Brooksii (Bateson). 
B. salmoneus (Giard). B. robinii (Giard). 

Balanoglossus represents an elongated vermiform body 
consisting of a distinct so-called anterior gland or pro- 
boscis, followed by a muscular collar and the gill 7'egion 
which gradually passes into the posterior portion of the 
body. Proboscis and collar serve as locomotory organs 
(boring themselves by peristaltic motions through the 
sand, which passes through the alimentary canal) and 
are therefore composed of external circular and internal 
longitudinal muscles. Cavities present in the two organs 
between the longitudinal muscles and the connective 
tissue cells communicate through dorsal pores with the 
outside, allowing the influx of water. On that account 
these cavities have been compared with the water- 
vascular system of the Echinodermata. Others maintain 
that they are simply excretory organs, which might by 
true on account of the (excretory ?) glandular structure 
present in the proboscis. The alimentary canal, begin- 
ning with the broad mouth opening immediately beneath 



276 BCHINODERMATA. 

the proboscis, extends in an almost straight line through 
the body. Of great importance are its diventricula. A 
dorsal evagination protrudes into the base of the proboscis. 
Between the ventral wall of the former and the epidermis 
of the latter the anterior part of the so called proboscis ■ 
skeleton is situated, whilst its two posterior branches, 
situated in the folds of the alimentary canal, clasp the 
foregut. Bateson and Kohler compare this structure to 
the chords of the Vertebrata. That portion of the ali- 
mentary canal which follows the collar region contains 
dorsally the branchial diverticula, usually situated along 
both sides of the median region and lined by a ciliated 
epithelium. They open through passages on the dorsal 
surface, and their position is easily recognized by the 
transverse arches connecting the individual pairs of 
diverticula ; a chitinous skeleton supports their walls. 
The water passes from the mouth through the foregut 
into the branchiae and through the pores outward. Some 
distance behind the branchial region the alimentary 
canal evaginates into hepatic diverticula, which cause a 
bulging out of] the external integument. Median dorsal 
and ventral mesenteris fasten the alimentary canal and 
divide the body-cavity into a right and left partition (in 
some cases fusing dorsally). The body cavity of the 
collar is different in structure and origin from that of the 
trunk,. being however, largely reduced by the accumu- 
lation of connective and muscular tissues, whilst the 
latter (maintained by Spengel and denied by Kohler) 
largely remains, its walls being composed of the longi- 
tudinal and circular muscles of the somatic and splanch- 
nic layers. 

The blood-vascular system consists of two principal 
vessels, one situated along the dorsal median line (current 



ECHINODERMATA. 277 

anteriorly) and the other along the ventral median line 
(current posteriorly); both give off branches at regular 
intervals to the body wall and^the organs. Kovalewsky 
maintains also the presence of two lateral vessels, which 
receive branches from the alimentary canal and the gills. 
The nature of the saccular structure at the floor of the 
proboscis, considered by some as the central organ of the 
blood system, is still disputed. The central nervous sys- 
tem consists of a thick cord, situated in the median 
region of the collar. Bateson maintains that it contains 
a central canal similar to that of the spinal cord of Verte- 
brates, others however affirm only the presence of irreg- 
ular interstices ; its interior fuses at the posterior (and 
interior) end with the epithelial cells of the body wall. 
This central cord gives off a strong nerve which extend 
along the dorsal median line of the body. It branches 
behind the collar and uniting below in the region of the 
first pair of branchiae, these nerves continue towards the 
posterior end of the body as the ventral median neive. 
The genital organs of Balanoglossus lie either within or 
back of the gill-region. The sexes are separate ; male 
and female organs are of the same form and occupy 
the same position in either sex ; they consist of simple 
or branched tubes lying along the side of the body and 
opening in rows along the dorsal surface. In some forms 
two median rows occur, situated between the branchial 
diverticula and the dorsal vessel. The portion follow- 
ing the branchial region is sometimes very highly devel- 
oped and differentiated as a special genital region. 

The genital products are ejected into the water through 
the ruptured body wall, after which fertilization takes 
place. Segmentation is total and nearly equal, result- 
ing in a blastula, followed by a typical invagination gas- 



278 ECHINODKRMATA. 

trula. Development is either direct from the egg to the 
adult, or it passes through a larval stage, whose form 
resembles the larvae of the Echinodermata. Joh. Miiller 
named it Totnarm. At an early stage an unpaired evagi- 
nation of the ccelom arises, which in its structure is said 
to correspond to the water- vascular vesicle of the Echino- 
derms, opening to the outside through a dorsal process. 
The proboscis, whose internal walls are formed by the 
so-called water-vascular vessel, has therefore been looked 
upon as the only remnant of the ambulacral tentacles. 
However, it is difficult .to carry the comparison any 
further; the characteristic ciliated band of the Echino- 
derms exhibits an entirely different arrangement in 
Tornaria. The presence of a vertex plate with its eyes 
and nerve cord rather suggests a relation of the latter to 
the trochophora. The existence of gills led to the as- 
sumption that Balanoglossus might belong to the Chor- 
data (Balfour). A comparison with Amphioxus showed 
certain relations with regard to the gills, the alimentary 
diverticulum with its skeletal body (chorda) and the 
mode of origin of the nervous system. A further re- 
markable resemblance exists between the anterior coelom 
sacs of Balanoglossus and the anterior coelom diverticula 
of Amphioxus, one of which enlarges and leads through 
a ciliated canal outward, thus corresponding exactly to 
the so-called water- vascular vesicle or anterior coelom sac 
of the Balanoglossus larva. However, all these phe- 
nomena do not furnish a sure basis for the above assump- 
tion (Korschelt and Heider). 



TUNICATA. 



A remarkable and in many respects isolated groups of 
marine animals of bilateral, saccular or barrel-shaped 
form, which owe their name to a gelatinous or cartilag- 
inous envelope or mantle completely surrounding the 
body. Since Kowalewsky's splendid discoveries, in 1866, 
the Tunicata have frequently been classified among the 
Vertebrates under the superclass of Urochorda on account 
of the presence of a notochord-like skeletal axis in the 
tail. Their respiration is pharyngeal; a distinct saccular 
heart occurs. The branch consists of both free and com- 
posite, fixed and free organisms. 

I Class: Ascidise. Mostly fixed form enveloped in a 
tunic of saccular shape. The inhalent (mouth) and exhal- 
ent pores are close together. The former, possessing in 
man}' cases four, six or eight marginal lobes, can be closed 
by a sphincter muscle ; the margin of the latter, situated 
dorsally behind the mouth, is often similarly divided and 
can also be closed. Branchial sac large, containing a 
circle of simple tentacles. Alimentary canal elongate, 
often causing a constriction of the body into thorax and 
abdomen. They either remain solitary, and are then of 
considerable size, or they form colonies without being 
enveloped in a common mantle; or numerous individuals 
live in a common mantle, arranged around a central 
opening, which leads into the openings of individual 
groups. 

1. Order: Larvacea. Small, free-swimming, pelagic 



28o TUNICATA. 

Ascidiatis, with large swimming tail containing a skele- 
tal axis (urochord). Branchial sac an enlarged pharynx 
with two ventral ciliated openings (stigmata); heart with 
two slits but no vessels. Anus opens ventrally in front 
of stigmata. No cloacal chamber. Nervous system con- 
sists of cerebral ganglion with three constrictions, and a 
long nerve cord running down to the tail along the left 
side of the urochord. They resemble the larvae of other 
Ascidians. Some have a shell-like covering. Repro- 
duction without budding or metamorphosis. Family: 
Appendiculariidcs , with short body, long tail and straight 
endostyle in Oikopleura (Appendicularid); or with long 
body (with anterior ectodermal hood), short tail and 
recurved endostyle in Fritillaria; or without heart, endo- 
style and intestine in Kowalevskia. 

2. Order: Ascidiacea. Sessile solitory or free swim- 
ming compound ascidians; adult without tail and noto- 
chord; with well-developed permanent test, increasing 
with the age of the individual. Branchial sac large and 
perforated by numerous stigmata which lead into the 
peribranchial cavity and open to the exterior by the 
atrial aperture. Reproduction . frequently by budding; 
in other cases embryo with tailed larva. 

I. Suborder: Ascidics simplices. Solitary forms large 
and surrounded by a very thick and hard cartilaginous 
mantle with protuberances of various kinds. The col- 
onies or social Ascidians have a common circulation, but 
each individual has its own test of a transparent hyaline 
consistency, i. Family: Clavellinidce, simple Ascidians 
which reproduce by budding to form colonies, the buds 
arising from a common branched stolon; body sometimes 
divided in three regions. Ecteinascidia . Clavellina. 
Perophora. 2. Family: Ascidiidce, solitary sessile Asci- 



ASCIDI^. 28l 

dians of considerable size; rarely aggregated, and then 
without common mantle and blood vessels; mantle par- 
enchyma gelatinous; branchial sac not folded, its aper- 
tures eight-lobed. Hypobythius. Ciona. Ascidia. Rhodo- 
soma. Abyssascidia. Corella. Corynascidia. C/ielyosoma. 

3. Family: Cynthiidce, solitary sessile, with leathery 
mantle parenchyma; branchial sac longitudinally folded, 
its aperture four-lobed. Styela. Bathyoncus. Cynthia. 
Boltenia. Culeolus. Cheveulius. 4. Family: Molgulidce, 
solitary forms, sometimes free; mantle parenchyma, in- 
crusted with sand; branchial sac longitudinally folded, its 
aperture six-lobed. Molgula. Eugyra. In A?iurella, 
direct development takes place. 

2. Suborder: Ascidicz composites. Sessile Ascidians 
enclosed in a common mantle layer and grouped around 
a common cloaca (Botryllus), forming a soft, bright- 
colored colony; mantle mass permeated by vascular 
canals. 1. Family: Distomidcz, divided into thorax and 
abdomen, vas deferens not spirally coiled. Distoma. 
Distaplia. Collella. Chondrostachys. 2. Family: Ccslo- 
cormidce, colony not fixed. Cozlocormus. 3. Family : 
Didemnidce, mantle-parenchyma with calcareous spicules, 
vas deferens spirally coiled. Didemnum. Leptochinum. 

4. Family: Diplosomidcs, mantle parenchyma reduced, 
rarely containing spicules ; vas deferens not spirally 
coiled. Diplosoma. 5. Family : Polychinidcz, zooids 
divided into thorax, abdomen and postabdomen, numer- 
ous testes. Pharyngodictyon. Polychinum. Aplidium. 
Amaroncium. 6. Family: BotryllidcB, most complete 
colonies, viscera of the simple undivided body side by 
side of the respiratory cavity, Botryllus. Botrylloides. 
7. Family: Poly sty elides, zooids not grouped in systems; 
branchial and atrial apertures four lobed. Thylacium. 
Goodsiria. Chorizocormus . 



282 TUNICATA. 

3. Suborder: Ascidice Salpiformes. Free -swimming 
pelagic colonies, shaped like a thimble or hollow cylinder 
closed at one end. The numerous individuals, of which 
these colonies are composed, are arranged at right angles 
to the long axis of the colony; the branchial apertures 
open on the outer surface, the atrial (exhalent apertures) 
on the inner into the common cloaca. The large bran- 
chial sac occupies the greater part of the body. The 
nerve ganglion contains a small pigmented sense organ. 
The alimentary canal is placed posteriorly to the bran- 
chial sac opening into a large peribranchial cavity, and 
the intestine and ovary being compressed together so 
that they resemble a nucleus. One ovum is matured at 
a time. In all these respects they resemble the Salps. 
Sexual reproduction and budding take place in^t~he same 
individual, the latter by means of a stolon, beginning at 
the posterior end of entostyle; it is very complicated. In 
sexual reproduction the embryo (cyathozooid) is developed 
within an ovarian sac, but produces by budding from a 
stolon a group of four ascidiozooids, from which the colony 
gradually increases. Only family: PyrosomidcE (name 
from phosphorescent glands on either side of the anterior 
end of the branchial sac). Pyrosoma. 

II Class. Thaliacea. Free swimming transparent 
pelagic forms, either simple or compound (usually in 
double rows) ; adult always without tail and notochord. 
The body is cylindrical or cask- shaped. Mantle apertures 
at opposite ends of the body. Mouth usually a trans- 
verse slit with movable lips. The band-shaped gill, 
situated in the large respiratory cavity has either two 
large (Salpa) or many small apertures (Doliolum), lead- 
ing to a single peribranchial cavity, into which the 
anus opens. Alimentary canal, heart and generative 



THALIAC3A. 283 

organs are closely packed together at the ventral pos- 
terior end, forming a brightly colored mass, the so-called 
nucleus. The nervous ganglion situated along the mid- 
dle of the dorsal margin sends off numerous nerves to the 
body and sense organs. Eight or nine circular muscle 
bands surround the body and effect rapid locomotion, 
which together with the highly developed nervous system 
makes this order far superior to the Ascidians. Alter- 
nation of generations occurs and may be complicated by 
polymorphism. 

1. Order: Hemimyaria. Body more or less fusiform, 
with the long axis antero-posterior and the branchial 
and atrial aperture nearly terminal. Anterior opening 
with a valve-like closable lips. Gill, a median band 
extending from the ganglion to the oval region. Free- 
swimming solitary forms alternate in their sexual con- 
dition with colonies budded from stolons and united in 
chains ; proterogynous. The single ovum develops into 
an embryo within the brood- pouch of the viviparous 
mother by means of a placenta, and becomes a single 
Salpa (asexual). 1. Family: Salpidce. Salpa. 2. Fam- 
ily: Octacnemidce . Octacnemus. 

2. Order: Cyclomyaria. Body cask-shaped, branchial 
and atrial apertures surrounded by lobes. Gill rather 
large, occupying the anterior dorsal half or more of the 
body, stigmata usually represented by two lateral rows 
in the posterior part only. Alimentary canal not com 
pressed into a nucleus. Ovaries with more than one egg. 
Testes and ovaria ripen at the same time. Complicated 
alternation of generations. Never form colonies. Fam- 
ily: DoliolidcB. Doliolum. Anchinia. 

Organization and development. The form of the body 
of the Tunicata exhibits in the various divisions such 



284 TUNIC AT A. 

remarkable modifications that a mutual relationship can 
scarcely be recognized. In the lowest forms the body 
consists of two divisions, the anterior enclosing the most 
important organs and exhibiting bilateral symmetry, and 
the posterior or tail-portion projecting ventrally at an 
angle from the latter and appearing like a simple append- 
age. In the highest Tunicates (Doliolum) a similar pro- 
cess is found only in the young which seems to prove 
that the ancestral type possessed such a tail-like region. 
The power of locomotion is lost with the propelling tail, 
the adult becomes sessile, and its external appearance 
simplified, whilst the internal structure becomes more 
complicated. A special chamber with the function of a 
cloaca arises into which the atrial aperture leads. The 
free-swimming Hemimyaria and Cyclomyaria propel 
themselves by means of their body walls, the orifices 
assume extreme anterior and posterior positions. The 
investment of the body consists, in its primitive condition, 
of a single layer of flattened cells formed from the ecto- 
derm and largely representing the body-wall. In the 
lowest Ascidians the cells around the inhalent orifice 
enlarge and secrete a gelatinous substance, which grad- 
ually envelopes a part or the whole of the body 
serving as a protecting envelope or test. In the higher 
Tunicata a firm cuticular secretion is produced by the 
whole body surface, varying from the softness of jelly 
to the hardness of cartilage and assuming the characters 
of connective substance, in which cellulose is embedded. 
This is the outer mantle. Complications of the mantle 
arise through the entering of numerous blood-vessels 
(Ascidia, L ), and in Chevreulius by the development of 
two valves, movable like shells. In colonies only mantle 
may enclose all the individuals. Also numerous other 



TUNICATA. 285 

integmentary formations may arise, such as unicellular 
gland, hair-like processes, calcareous spicules, etc. The 
axial organ present in the tail of the Appendicularise 
may be looked upon as a skeleton, composed of cells 
which secrete a homogeneous elastic chord enclosed in a 
continuous sheath. This organ is homologous (in posi- 
tion) with the chorda dorsalis of the Vertebrata, a fact 
which justifies the classification of the Tunicata together 
with the Vertebrata under the common head of Chordonii 
(Hatschek). The muscular system is chiefly developed 
around the respiratory cavity, consisting in the Ascid- 
ians of three layers, an external and internal longi- 
tudinal, and an internal circular layer; in the Salps 
of band-like rings of muscles embedded in the body 
wall, serving also functions of locomotion; in the-Ap- 
pendicnlarise they are confined to a dorsal and ventral 
band of the tail. The central nervous system arises from 
a differentiation of the ectoderm and occupies a dorsal 
position, which distinguished the Tunicata from all other 
invertebrate animals. It consists of an anterior ganglion, 
divided into three consecutive lobes, produced by an un- 
equal thickening of the wall of the original ectodemal 
tube; the foremost is (in Ascidiae and Salpae) In close con- 
nection with the origin of the visual organ. Peripheral 
nerves pass out from the anterior ganglion, encircling 
the inhalent aperture; others pass to the muscles, viscera 
and sense organs. In the larval Arcidians and the 
adult Appendiculariae the nervous system is made up of 
an anterior elongated ganglion with three dilatations 
and a chord which passes posteriorly to the end of the 
tail, where it forms three ganglionic swellings, the first 
of which is most constant and regarded as the central 
organ of the nervous system. With the atrophy of the 



2$6 YUNICAYA. 

tail the caudal division of the nerve-centre disappears 
and the anterior portion becomes more elaborate. The 
lobes of the oval and atrial apertures and the tentacles 
serve as organs of sense. A more differentiated organ of 
sense is the so called ciliated groove, clothed with flagel- 
late cells and situated either in front of the ganglion or on 
that surface of the ganglion which is turned towards the 
branchial chamber; it is considered as an olfactory organ. 
An auditory vesicle is present on the left side of the gang- 
lion (paired in Pyrosoma). Eye spots have been observed 
on the lips of the large openings in the simple and com- 
pound Ascidians. In Salpa the eye is represented by a 
spherical process with a brownish- red pigment spot and 
numerous rod-shaped structures. The alimentary canal 
(arising from the entoderm) forms the part of the body 
which is most peculiar in the Tunicata. The anterior 
portion of the foregut is transformed into a respiratory 
organ similar to that of the Knteropneusta. The anterior 
mantle-opening or mouth leads into this cavity, admit- 
ting the water which not only brings nutrient matter, 
but serves also for respiration, finding its exit through 
special openings (stigmata arranged metamerically) in 
the wall of the pharynx (sometimes a special peribran- 
chial chamber surrounds the latter) whilst the food par- 
ticles are directed to the alimentary canal proper by 
special arrangements. The special form of the respira- 
tory organ presents numerous modifications of systematic 
value (comp. classif.). A ciliated groove bounded by two 
folds extends along the middle ventral line of the latticed 
branchial sac (in Ascidians) between the mouth and the 
oesophagus. The glandular walls of this groove are 
called the endostyle. Near its foremost extremity ciliate 
tracts commence which embrace the entrance to the res- 



TUNICATA. 287 

piratory chamber and continue dorsally either to the 
oesophagus or to the neighborhood of the great ganglion, 
taking a spiral turn on the way or ending in a ciliated 
groove, (similar organ in Balanoglossus). The alimen- 
tary tract succeeding the respiratory cavity consists of a 
ciliated funnel-shaped oesophagus, of a stomach usually 
provided with hepatic diverticula and of a small intestine 
forming a loop and opening into the cloacal cavity. The 
blood-vascular system of the Appendiculariae consists of a 
heart, a short sac with its end attached to two cells whilst 
its delicate walls present two longitudinal slits placed on 
opposite sides. In the others vascular trunks (lacunae) 
pass from the heart into a system of spaces in the body 
wall through which the blood passes (of special compli- 
cation in the Ascidians). Of great importance are the 
two longitudinal vessels, one in the median dorsal line 
and the other beneath the ventral groove, both connected 
by transverse anastomosing channels placed in the wall 
of the branchial cavity and communicating with a num- 
ber of lacunae in the respiratory organ, over whose sur- 
face the water is continually renewed. Thus it appears 
that a portion of the primitive body cavity (a modified 
form of enterocoelom) is taken up by the blood vessels. 
Peculiar to all Tunicata is the alternating direction of the 
blood- current, produced by the heart (compare Phoronis), 
so that we cannot speak of an arterial and a venous 
division of the vascular system. Organs of excretion are 
only present to a very limited extend, a tubular glandu- 
lar organ without openings has been found near the 
branchial chamber or further back in many Ascidians. 

Only the Appendiculariae possess sexual organs uni- 
versally; a large number of the other Tunicata is devoid 
of such on account of the frequent asexual reproduction, 



288 TUNtCATA. 

which resembles the process of multiplication by bud- 
ding. All the members of this branch are hermaphro- 
dite; the genital glands, which may be either simple or 
complicated, are situated among the viscera in the anter- 
ior part of the body. Male and female organs may ripen 
their products at different times. Fertilization and em- 
bryonic development usually takes place in the cloacal 
chamber, the embryo within the egg membrane either 
leaving early through the exhalent aperture, or at later 
stage of development, being meanwhile nourished by a 
sort of placenta. 

Colonies are formed in various ways. Those of the 
Ascidiana are produced by means of a stolon sent forth 
by the parent, and composed of ectodermal and ento- 
dermal elements. It may directly grow into an organi- 
zation similar to the parent, or it may produce a second 
bud, both together developing and producing each a new 
bud, and so on until eight buds are formed, when the 
mother dies and the young animals are left in communi- 
cation with one another by means of a cloaca, constitut- 
ing a rosette-shaped group. Sexual organs are only 
found in the colonies; persons developing from an egg 
never acquire such organs in the compound Ascidians. 
In the higher Tunicata colonies are formed by a special 
organ, situated either upon the dorsal surface (Cyclo- 
myaria) of the body, or ventrally within a cavity (Hem- 
imyaria), and called the germ-stock or stolo prolifer. 
In the former the buds arise from the stolon by short 
processes; in the latter the stolon becomes a part of the 
new outgrowth. An alternation of generations arises 
here likewise, inasmuch as the forms provided with a 
stolon are always permanently devoid of sexual organs, 
whilst the individuals derived from the stolo prolifer are 



TUNICATA. 289 

sexual. In Doliolum, however, several generations suc- 
ceed one another in the cycle of development. 

The embryonic development of the Ascidians resembles 
that of the lower Vertebrates, especially of Amphioxus. 
Segmentation is total and equal, resulting in a spherical 
blastula which changes into an invagination gastrula, 
from the ectoderm of which the neural tube is devel- 
oped. The axial skeletal structure arises from a double 
row of entoderm cells (like the chorda dorsalis). Ali- 
mentary canal, nervous system and notochord occupy an 
analogous position to those of the Vertebrates. The 
movable larva (Ascidian tad-pole) # swims about freely, 
often producing a colony before becoming fixed. It 
possesses the essential characters of the Chordata, inas- 
much as it has a longitudinal skeletal axis (notochord) 
separating a dorsally-placed nervous system from a ven- 
tral alimentary canal. After it has fixed itself it under- 
goes a remarkable series of retrogressive changes, which 
convert it into the adult Ascidian. The important con- 
clusion drawn from all this is, that the Tunicata are the 
degenerate descendants of a group of Chordata. 



N 



VERTEBRAfA. 



Bilaterally symmetrical animals possessing an internal 
skeleton (vertebral column) which traverses the longitu- 
dinal axis of the body dividing it into a number of 
metameres (primitive vertebrae). This skeleton sepa- 
rates a dorsal from a ventral division of the body; its 
dorsal processes (upper vertebral arches) enclose the 
nervous centres (brain and spinal cord), and its ventral 
processes (ribs), the digestive canal continued from a 
respiratory chamber and with its differentiations em- 
bedded in a ccelom. There are at most two pairs of 
limbs. 

*I Superclass: Cephalochorda. No anterior dilatation 
of the nerve tube to form a (traces of a brain differ- 
entiation in Amphioxus have recently been observed by 
Hatschek, predicating undoubted similarities to the 
higher Vertebrates) brain and no specialized skeletal 
brain-case, notochord, a simple rod, extending from one 
extremity of the elongate body to the other, ending 
beyond the nerve-cord. 

I Class: Acrania. Walls of the body a series of 
muscular segments (myotomes) not in any way fused to 
form a head or cranial structure; no jaws nor extremities; 
pharyngeal walls fissured, heart a longitudinal vessel, 
which gives off branchial vessels, uniting into an aorta; 
a liver and vena cava present. 

^Compare E. D. Cope's "Paleontology of the Vertebrata." 1891. 



PISCES. 291 

Only Order: Leptocardii. Somewhat asymmetrical. 
Pharyngeal fissures enclosed externally by a fold of the 
integument, which forms a chamber (atrium) with in- 
ferior openings of the alimentary canal at opposite ex- 
tremities. Amphioxtis. 

II Superclass: Craniata. Tubular cerebro-spinal 
nerve-mass, swollen anteriorly to form a brain, consisting 
primarily of three successive vessels, from the anterior 
of which olfactory organs and the eyes originate. Car- 
tilaginous cranium or brain-case round the anterior ex- 
tremity of nerxe cord, enlarging laterally to protect the 
brain. The cartilage is in most cases replaced by bone. 

I Class: Agnatha. No lower jaw nor pectoral arch. 
Internal skeleton cartilaginous. Largest number extinct. 

1. Subclass: Ostracophoii. An osseous dermal skeleton 
with lateral limblike appendages. Extinct. 

2. Subclass: Marsipobranchii. Vermiform. No osse- 
ous skeleton nor lateral limblike appendages. No ex- 
tinct species known. 

1. Order: Hyperotreti. Branchial fissures communi- 
cating directly with the pharynx ; nasal sac perforating 
the palate. Myxinidce. (Hags.) Bdellostomidce. 

2. Order: Hyperoarti. Branchial Assures communi- 
cating with a common branchial passage which opens 
into the pharynx ; nasal sac not perforating palate. 
Petromyzon tides . (Lampreys) 

II Class: Pisces. Lower jaw and pectoral arch 
present. Basicranial axis not ossified ; vertebral column 
consisting chiefly of intercentra. Limbs represented by 
many radiate fins, which are also present on the median 
lines of the body; a coracoid bone; heart with two 
chambers ; cold-blooded ; breath by means of gills ; one 
or no occipetal condyles; no internal nares. First two 



2 £2 VERT^BfcATA. 

subclasses without, second two with suspensorium of the 
mandible. 

i. Subclass: Holocephali. No dermal cranial ossifica- 
tions ; ventral claspers ; no opercular bones ; small oper- 
cular membrane ; no maxillary arch. 

Only order: Chimceroidei. (Sea-cats.) A single ex- 
ternal branchial fissure ; actinotrichia present ; basilars, 
axonosts, and neural spines articulating with each 
other ; pectoral fin pleuribasal, with three axonosts and 
and numerous basilars ; ventrals with elongate axonosts 
and basilars. One pair of large teeth in the upper and 
two in the lower jaws. Chimcera. Callorhynchus . 

2. Subclass: Dipnoi. Scaly fishes with branchial and 
pulminary respiration, muscular conus arteriosus and 
spiral valve in the intestines. 

i. Order: Arthrodira. Notochord persistent. Paired 
fins rudimentary or absent, pelvic elements double, 
lateral, body more or less protected by plates. Extinct. 

2. Order: Sirenoidea. Paired fins unibasal; pelvic ele- 
ments fused on the middle line; body without bony plates- 
Ceratotidce. LepidosirenidtE. 

3. Subclass: Elasmobranchii (Sharks). Cartilaginous 
fishes with large pelvic and pectoral fins, transverse 
mouth on the ventral side, with five pairs of branchial 
organs and branchial slits; muscular conus arteriosus 
with several rows of valves. Intestine with spiral valve. 

1. Order: Ichthyotomi. Persisting notochord; paired 
fins unibasal and archipterygial; a basioccipital element. 
Extinct. 

2. Order: Selachii (Sharks and Rays). Paired fins pleu- 
ribasal; no basioccipital. 

1. Suborder: Tectospondyli. Body suppressed with 
branchial fissures, either ventral or lateral concentric 



PISCES. 293 

laminae of the vertebrae predominating over the radiating 
laminae; anal fin absent. Teeth in the usual jaws. Peta- 
lontidae. Psammodontidae. Rajidae. Rhinobatidae. Try- 
gonidae. Myliobatidae (eagle rays). Spinacidae. Teeth 
in a produced muzzle: Pristiophoridae. Pristidae (saw- 
fishes) . 

2. Suborder: Asterospondyli. Radiating laminae of 
vertebrae predominating over the concentric; anal fin 
present. Teeth molariform: Cestraciontidae, teeth sepa- 
rate. Cochliodontidae, teeth confluent. Teeth with ele- 
vated cusps: Hexanchidae. Scylliidae. Lamnidae. Car- 
chariidae. 

4. Subclass: Teleostonri (true fishes). Fishes with bony 
skeleton, with free gills (four on each side) and an ex- 
ternal branchial operculum. Bulbus arteriosus with two 
valves at its base. Optic nerves do not form a chiasma. 

1. Superorder: Rhipidopterygia. Median fins each with 
a single bone representing axonosts. Paired fins unibasal. 
Extinct. 

2. Superorder: Crossopterygia. Median fins with numer- 
ous axonosts. Paired fins with baseosts, pectoral axo- 
nosts distinct from baseosts. Only living famity: Polyp- 
teridae. 

3. Superorder: Podopterygia. Median fins with numer- 
ous axonosts. Paired fins with baseosts; pectoral axonosts 
and baseosts confounded; pluribasal. 

1. Order: Lysopteri. Branchiostegal rays present. 
Extinct. 

2. Order: Chondrostiei. No branchiostegal rays; de- 
generate representatives of the Podopterygia, deficient 
in various normal ossifications, middle line of the skull 
covered by dermal bones. Accipenseridae fsturgeons). 
Polyodontidae (paddle-fishes). 



294 VERTEBRATA. 

4. Superorder: Adinopterygia. Median fins with nu- 
merous axonosts; pectoral fins only with baseosts, con- 
founded with axonosts, and pluribasal. They represent 
the finally specialized type of the true firsh. Skeletal 
parts of the true fins abbreviated, basilar elements sessile 
on the scapular; rays of the median fins distinctly devel - 
oped and articulated; tails usually homocercal; vertebrae, 
in most cases ossified. 

1. Tribe: Malacopteri. Ventral fins abdominal; a 
ductus pneumaticus; no spinous dorsal fin; parietal bones 
not usually separated by supra occipital; scales usually 
cycloid. Represented by sixteen orders. 

2. Tribe: Aconthopteri. Ventral fins usually thoracic 
jugular; no ductus pneumaticus; usually a spinous dorsal 
fin; parietal bones usually separated by the supra occi- 
pital; scales usually ctenoid. Represented by ten orders. 

Ill Class: Batrachia. L,ower jaw and pectoral arch 
present. Basicranial axis not ossified; vertebral column 
consisting chiefly of intercentra. L,imbs consisting of 
one basal element, two propodials, and metapodials and 
digits; no median fins; lower jaw attached to a suspen- 
sorium, complex; no opercular bones; a caracoid; heart 
with three chambers; two occipital condyles; internal 
nares. 

1. Subclass: Stegocephali. Basioccipital, supra-occi- 
pital, intercalary, and supra-temporal bones present; 
propodial bones distinct- Extinct. 

2. Subclass: Urodela. Basioccipital, supraoccipital, 
and supratemporal bones wanting, propodial bones dis- 
tinct; no urostyle. 

1 . Order : Proteida. An os intercalare. Palatine arch 
and vomer present. Family : Protidce, anterior limbs short 
with three digits ; hind limbs with two digits. Proteus. 



MONCONTYLIA. 295 

2. Order: Pseudosauria. A maxillary arch and 
vomers ; no os intercalare. Cryptobranchidae. Ambly- 
stomidae. Salamandridae. Amphinmidae. 

3. Order : Trachystomata. No os intercalare ; no 
maxillary arch or vomers. Sirenid<z. Rudimentary 
anterior limbs, without posterior limbs. Siren. 

3. Subclass : Salientia. Basioccipital, supraoccipetal, 
intercalare and supratemporals wanting ; frontals and 
parietals connate ; propodial bones connate ; lumbosacral 
vertebrae united into a urostyle. 

Only order: Anura. Vomers and palatopterygoid 
arch present. 

1. Aglossa. Internal nostrils opening together on the 
middle line; no tongue; carocoids connected by a carti- 
lage on each side. Xenopidae. Pipidae. 

2. Arcifera. Internal nostrils separate; a tongue; 
coracoids connected by a separate cartilage on each side, 
one overlapping the other. Ten families : Bufonidae. 

3. Suborder: Firmisternia. Internal nostrils sepa- 
rate ; a tongue ; a single median cartilage connecting all 
the coracoids ; scapular arch free. Eight families, 
Ranidae. 

4. Suborder : Gastrechmia. As in the preceding, but 
scapular arch articulated to skull. Hemisidae. 

IV Class : Moncondylia. Basicranial axis ossified; 
vertebral column chiefly of centra ; an amnion and 
alantois. Limbs as in Bratrachia ; one occipital condyle: 
a suspensorium of the lower jaw; mandible segmented ; 
ankles between first and second rows of carpal and tarsal 
bones ; heart with three or four chambers. 

1. Subclass: Reptilia. Anterior limbs ambulatory, 
with numerous carpal and metacarpal bones; two aorta 
roots; integument consisting partly of scales. In eight 



296 VERTKBRATA. 

out of the nine orders the quadrate bone is united with 
the postorbital bars by a suture; in the ninth it is loosely 
articulated with the postorbital bars and only proximally. 

1. Order: Ichthyopterygia, extinct. 

2. Order: Testudinata. No supratemporal; sub- and 
post-pelvic ossifications; interclavic and clavicles sepa- 
rated from and below scapular arch; ribs oneheaded; 
caracoid large, free. A paroccipital bone. Suborders: 
Athec<z, leather back turtles. Trionychoidea, soft or mud 
Tortoises. Cryptodira, Turtles. Land Tortoises. Pleu- 
rodira, southern fresh water Tortoises. 

3. 4, 5, and 6. Orders: (T/ieromora, Plesiosauria, Orni- 
thosautia, Dinosauria) extinct. 

7. Order: Crocodilia. Cranium with two postorbital 
bars; no paroccipital bone. Suborders: Parasuchia and 
Pseudosuchia extinct. Eusuchia. Nareal canal under- 
roofed to behind larynx; no clavicle; pubis excluded 
from acetabulum; external nostrils anterior. True croc- 
odiles. 

8. Order: Rhynchocephalia. Cranium with two postor- 
bital bars; no paroccipital bone. Ribs one-headed; an 
interclavicle; acetabulum closed; feet ambulatory. Sub- 
orders: Sphenodontina. One species of Sphenodontidse 
living in New Zealand. Choristodera. Extinct. 

9. Order: Squamata. Quadrate bone in contact only 
with adjacent elements; no paroccipital; supratemporal 
present; ribs one-headed; one or no postorbital bar. 
Suborders: Lacertilia (Lizards). Ve^ numerous, eleven 
superfamilies with twenty-five families. Pythonomorpha, 
extinct. Ophidia (Snakes) with six superfamilies and 
nineteen families. 

2. Subclass: Aves. Anterior limbs volant, with the 
carpels and metacarpels more or less coossified and re- 



t MAMMALIA. 297 

duced in numbers; integument consisting partly of 
feathers; one aorta root. 

1. Superorder: Sauturte. Metacarpal and carpal bones 
all distinct, the digits with ungues; caudal vertebrae 
numerous unmodified; clavicles united; pelvic elements 
distinct; teeth present. Ornithopappi with Archseopteryx, 
extinct. 

2. Superorder: Eurhipidiu ce . Metacarpal and carpal 
bones reduced in number, coossified; ungues wanting or 
single; caudal vertebrae reduced in number, the terminal 
areas usually coossified. 

1. Tribe: Ratitcz. Ischium usually free from ilium pos- 
teriorly; palate dromaeognathous (i. e., maxillopalatines 
articulating with vomer, which is between them, pala- 
tines not articulating directly with sphenoid rostrum); no 
teeth. Four orders, two of which are living. Struthiones 
(Ostrich). Sternum without keel; clavicles; wings rudi- 
mental. Apteryges (Kiwis). Sternum without keel; no 
clavicles; wings rudimental. 

Second and third tribes {Odontolccz, Odontotormae) are 
extinct. 

4. Tribe: Euornithes. Ischium coossified with ilium 
posteriorly, palate not dromaeognathous; feathers dis- 
tributed in area, those of the wings much differentiated. 
Fifteen suborders of birds with numerous families belong 
here. 

5. Tribe: Impennes. Ischium coossified with ilium; no 
teeth; feathers universally distributed and not differen- 
tiated on wings. .Only order: Ptilopter with the family 
Aptenodytidae (penguins). 

V Class: MAMMALIA. Basicranial axis ossified; 
vertebral column of centra; an amnion and allantois. 
Limbs as above; two occipital condyles; no suspensorium 

N* 



298 VERTKBRATA. 

of the lower jaw; mandible not segmented; ankles be- 
tween propodial bones and carpus or tarsus; heart with 
four chambers; warm-blooded. Viviparous, suckling 
their young with the secretion of milk glands (mam- 
maria). 

1. Subclass: Prototheria. An interclavicle ; a large 
caracoid articulating with the sternum. Two orders 
extinct. Only living order : Monotremata. Jaws elon- 
gated forming a beak ; no true teeth at maturity; the 
feet are short, five toed, and furnished with strong claws; 
oviparous. Ornithorhynchidse. No extinct species 
known. 

2. Subclass : Eutheria. No interclavicle ; caracoid 
very small, coossified with scapula ; not reaching ster- 
num. 

1. Section: Didelphia. Marsupial pelvic bones; palate 
perforated ; vagina double ; placenta wanting ; corpus 
callosum rudimental ; cerebral hemispheres small. 

1. Order: Marsupialia. (Kangaroos. Opossums.) 
One deciduous molar tooth. Pouch or marsupium en- 
closes the mammary glands and receives the helpless 
young after birth. 

2. Section: Monodelphia. No marsupial bones; palate 
generally entire ; one vagina ; placenta and corpus cal- 
losum well developed. A. Mutilata. Posterior limbs 
wanting, or represented by minute rudiments ; anterior 
limbs oarlike. 

2. Order : Cetacea. (Whales.) Elbow joint inflexible ; 
carpal discoid, and, with the phalanges separated by 
cartilage ; lower jaw without ascending names ; posterior 
limbs absent. Represented by three orders: Atch&oceti. 
Odontoceti. Mysticeti. 

3. Order : Sirenia. Elbow joint flexible ; carpals 



MAMMALIA. 299 

and phalanges with close articulations ; mandible with 
ascending ramus. Incisors replaced ; grinders flat, no 
canine teeth. Sirenia. 

B. Unguiculata. Posterior limbs present ; unequal 
phalanges compressed and curved on one or all the feet. 
Carpal and tarsal bones generally in linear series. 

4. Order : Edentata. Teeth without enamel ; no 
incisors ; limbs ambulatory ; hemispheres small. Ver- 
milinguia (Ant-eaters), Dasyopoda (Armadillos), Brady- 
pod a (Sloths). 

5. Order: Glires. Teeth with enamel; incisors pres- 
ent. No postglenoid process; mandibular condyle not 
transverse; mastication proal: with freely movable 
clawed digits; hemispheres small. Suborders: Hybi- 
comorpha (Porcupines, Cavies, etc.). Sciuromorpha (Bea- 
ver). Myomorpha (Mice, Rats, etc.). Lagomorpka (Rab- 
bits). 

6. Order: Chiroptera. Teeth as above. Anterior 
limbs volant; hemispheres small. Suborders: Amimali- 
vora (insectivorous Bats). Frugivora (frugivorous Bats). 

7. Order: Bunotheria. Teeth as above. A post- 
glenoid process; mandibular condyle transverse; masti- 
cation orthal; no scapholunar bone; hemisphere usually 
small, smooth. Four suborders. One living, hisec- 
tivora (Hedgehogs, Shrews, Moles). 

8. Order: Carnivora. Teeth as above. A post- 
glenoid process; limbs not volant, with a scapholunar 
bone; mastication orthal; hemispheres larger, convoluted. 
With strongly clawed digits. Two suborders: Fissipedia, 
digits distinct; posterior limbs free. Bear. Badger. 
Skunk. Sea-otter. Dog. Civet. Hyena. Cat. Pinnipedia, 
digits united into paddles by integument; hind limbs 
partly enclosed in general integument. Seals. Walruses. 



300 VERTEBRATA. 

9. Order: Ancylopoda, Carpal and tarsal bones alter- 
nating; faceted. Anterior limbs prehensile; mandibular 
condyle and mastication transverse. Extinct. C. Un- 
gulata. Posterior limbs present; ungual phalanges not 
compressed and hooked. 

10. Order: Taxeopoda. Carpal and usually tarsal 
bones in linear series. L,imbs ambulatory; teeth with 
enamel. Suborders: Quadrumana (lemurs). Anth?o- 
pomorpha (Orang-outang. Gorilla. Man). 

11. Order: Toxodontia. Carpal bones alternating ex 
ternally; tarsals in linear series; limbs ambulatory, me- 
dian digits longest; teeth with enamel. Extinct. 

12. Order: Proboscidea. Tarsal bones alternating; car- 
pels linear or reversed diplarthrous. Cuboid bone partly 
supporting navicular, not in contact with astragalus; no 
canine teeth. Long proboscis serving as prehensile 
organ. Elephantidse. 

13. Order: Amblypoda. Both tarsal and carpal series 
more or less alternating; the distal row inwards. Os 
magnum not supporting scaphoides; cuboid supporting 
astragulus; superior molars tri tubercular. Extinct. 

14. Order: Diptarthra. Tarsal and carpal series as 
in preceding; os magnum supporting scaphoides molars 
quadritubercular. The most specialized type of Mam- 
malia in regard to structure of skeleton dentition and 
digestive system, but inferior to the anthropomorphous 
Taxeopoda in the structure of the brain. Suborders: 
Perissodactyla (Rhinoceros. Tapir. Horse). Artiodactyla 
(Swine. Boar. Hippopotamus. Dromedary. Camel. 
Giraffe. Musk Deer. Antelope. Gnu. Sheep. Goat. 
Ox; and allies). 

Inasmuch as the anatomy of the Vertebrates receives 
special attention in all the smaller or larger text-books, 



VERTEBRATA. 301 

on Practical Zoology, and is frequently taught as a sep- 
arate branch, we considered it sufficient to confine our- 
selves here to a few remarks on the reproduction, devel- 
opment and phylogeny of this branch. 

Reproduction is always sexual, and separate sexes are 
the rule (except in Serranus scriba and partly in Bufo 
variabilis). The sexual organs are generally paired, 
opening either directly from the body cavity through 
genital pores to the exterior, or indirectly through 
paired ducts. These often unite in the lower Verte- 
biates, forming an unpaired canal which opens into the 
cloaca, or they are highly differentiated, and connected 
with accessory glands and copulatory organs, most com- 
plicated in the Mammalia. The large majority of Pisces, 
Batrachia and Monocondylia are oviparous, whilst all the 
Mammalia are viviparous. 

The development of the embryo begins either with a 
total equal, or unequal or discoidal segmentation (p. 
43), which usually results in a disc-like embryonic for- 
mation or blastoderm lying upon the yolk. The pos- 
terior end of the disc develops into the alimentary 
cavity, whilst the layers of the blastoderm thicken 
and form a primitive streak marking the long axis of the 
embryo. An ectodermal groove arises along the dorsal 
surface, growing together along the edges and forming 
an epithelial free tube (medullary) which gives rise to 
the spinal cord and the brain, and beneath which the 
entodermal notochord develops. The mesoderm ex- 
tends along the sides of these structures, forming two 
bands, the median portions of which become gradually 
segmented (proto vertebral plates), developing into the 
proto-vertebrse. Between these and the unsegmented 
lateral plates the archinephric duct separates off, while 



302 VERfEBRATA. 

the genital glands arise from the peritoneum of the 
lateral mesodermal plates. In the meantime the ali- 
mentary canal has become further developed on the 
ventral side of the blastoderm, gradually absorbing the 
yolk and usually leaving a yolk sac. Metamorphosis 
occurs only in the naked Salientia and several Fishes. 
In the Monocondylia and Mammalia the outer layer of 
the blastoderm is raised at the anterior and posterior end 
of the embryo, and forms two folds covering the head 
and tail end. These folds gradually fuse over the body 
and form a closed sac filled with fluid {amnion). An- 
other organ characteristic of the higher Vertebrates is 
the allantois correlated with the disappearance of branchial 
respiration and the complete absence of a metamorphosis. 
It arises at the posterior end of the body as a vesicular 
evagination of the ventral wall of the alimentary canal 
and grows into a large vascular sac filled with fluid, and 
representing an embryonic respiratory organ. 



INDEX. 



Acantharia, 20. 

Acanthocephali, 108. 

Acarina, 193. 

Acoela, 96. 

Acotylea, 95. 

Actinaria, 85. 

Actinopterygia, 294. 

Aculeata, 188. 

Adambulacral, 267. 

Adductor muscle, 248. 

Agassiz, Iv., 10. 

Agnatha, 291. 

Allantois, 302. 

Alloioccela, 96. 

Alternation of generation, 70. 

Alveoli, 269. 

Amblypoda, 300. 

Ambulacra, 265. 

Amnion, 302. 

Amoeboid cells, 45. 

Amcebina, 19. 

Amphineura, 222. 

Amphipoda, 173. 

Ampulla, 267. 

Analogy, 13. 

Anatinacea, 234. 

Ancyclopoda, 300. 

Anisopoda, 172. 

Annulata, 108. 

Anomura, 176. 

Antennae, 196. 

Antennata, 180. 

Anthozoa, 85. 



Anthropotnorpha, 300. 
Anura, 293. 
Aplacophora, 222. 
Apoplasmatic, 45, 59. 
Aposcolecida, 135. 
Appendiculariidae, 280. 
Apterygota, 183. 
Arachnoidea, 189. 
Araneidea, 192. 
Archhydra, 88. 
Archiannelida, no. 
Archiblastula, 38. 
Architaenioglossa, 225 
Archoplasmic, 32. 
Aristotle, 8. 
Aristotle's lantern, 269. 
Arthrodira, 292. 
Arthrostraca, 172. 
Articulata, 257. 
Ascidise, 278. 
Ascidiacese, 280. 
Ascoglossa, 228. 
Asconidae, 78. 
Assimilation, 7, 62. 
Asteroidea, 262. 
Astracoda, 168. 
Astropectinidea, 263. 
Auditory organs, 52, 53. 
Auricularia, 274. 
Aves, 296. 
Autoplasmatic, 45. 
Axis cylinder, 48. 
Axopodia, 23. 



3<H 



INDEX. 



Azygobranchia, 224. 
Baer, C. E. v., 10. 
Balanoglossus, 275. 
Basal membrane, 61. 
Basommatophora, 226. 
Bateson, 276. 
Batrachia, 294. 
Bilateria, 41. 
Biogenesis, 15. 
Biology, 12. 
Bivium, 266. 
Blastoccelum, 36. 
Blastoderm, 36. 
Blastoidea, 258. 
Blastopore, 36. 
Blastosphere, 36. 
Blastozoa, 12. 
Blastula, 35. 
Blood circulation, 64. 
Bojanus, organ of, 253. 
Bothriocephalus, 97. 
Brachiolaria, 273. 
Brachiopoda, 115. 
Brachyura, 176. 
Branchiata, 167. 
Branchiopoda, 168. 
Branchiura, 169. 
Brisingidea, 263. 
Bristles, 46. 
Bryozoa, 114. 
Budding, 70. 
Bunotheria, 299 
Byssus, 246. 
Calcaria, 77. 
Calcispongise, 77. 
Campauaria, 82. 
Capitellidse, no. 
Cardiacea, 233. 
Carnivora, 299. 
Cartilage, 61. 



Caryokinesis, 32. 

Catallacta, 22. 

Categories of the System, 16. 

Cell, 5, 46, 48. 

Cellulose, 24. 

Cenogenetic, 15. 

Centrolecithal, 40. 

Cephalochorda, 290. 

Cephalopoda, 234. 

Cephalothorax, 169. 

Ceraspongiae, 78. 

Cestoda, 97. 

Cetacea, 298. 

Chaetoguatha, 116. 

Chsetopoda, 109. 

Chelicerse, 216. 

Chilipoda, 181. 

Chiinaeroidei, 292. 

Chiroptera, 299. 
Choanoflagellata, 21. 
Chondrastei, 293. 
Chordata, 278. 
Chorion, 31. 
Chromosome, 33. 
Chromatophere, 241. 
Chun, 93. 
Cidaridea, 259. 
Cilia, 45. 
Cirripedia, 169. 
Cladocera, 168. 
Claus, 204. 
Clavulae, 267. 
Clypeastridea, 259. 
Cnidaria, 49, 81. 
Ccelenterata, 76. 
Cceloblastula, 38. 
Ccelum, 42. 
Cceloplana, 103. 
Coleoptera, 187. 
Collembola, 183. 



INDEX. 



305 



Connective tissue, 61. 
Cope, 12, 290. 
Copepoda, 168. 
Copulation, 67. 
Corals, 85. 
Cormus, 29. 
Corpus vitreum, 56. 
Corrodentia, 184. 
Cotylea, 95. 
Craniata, 291. 
Crevettina, 173. 
Cribrellum, 217. 
Crinoidea, 256. 
Crossopterygia, 293. 
Crustacea, 167. 
Crocodilia, 296. 
Crystaline style, 247. 
Ctenidium, 237. 
Ctenophora, 87. 
Cubomedusae, 86. 
Cumacea, 174. 
Cuticula, 61. 
Cuvier, 9. 
Cyclomyaria, 283. 
Cyphophthalmidea, 191. 
Cystidea, 258. 
Darwin, Charles, 11, 15, 71. 
Decapoda, 175. 
Delaminatiou, 40. 
Dentomerit, 24. 
Dermaptera, 183. 
Desor, 124. 
Dibranchiata, 235. 
Dicyetnida, 76. 
Didelphia, 298 
Differentiation, 71. 
Dimyaria, 248. 
Dinoflagellata, 21. 
Dinophilus, 116. 
Diotocardia, 223. 



Diplopoda, 182. 
Dipneumona, 264. 
Dipnoi, 292. 
Diptera, 189. 
Discoidal, 38. 
Discomedusse, 87. 
Disconanthae, 84. 
Docoglossa, 224. 
Dysasteridae, 260. 
Ecardines, 116. 
Echinodermata, 256. 
Echinasteridea, 262. 
Echinometridae, 259. 
Echinoidea, 258. 
Echiuridse, ill. 
Ectoderm, 66. 
Ectoparasitica, 96. 
Ectoprocta, 114. 
Edentata, 299. 
Ederioasterida, 258. 
Elasmobranchii, 292. 
Embryology, 13. 
Embryonic division, 69. 
Enteropneusta, 275. 
Entoderm, 66. 
Entomostraca, 167. 
Entoparasitica, 97. 
Entopodite, 195. 
Entoprocta, 114. 
Entostyle, 286. 
Epheineridea, 184. 
Epimerit, 24. 
Epitheliogenous, 44. 
Epithelium, 44, 50, 51. 
Epitheloids, 44. 
Errantia, III. 
Eucharis, 93. 
Eucopepoda, 168. 
Eudarina, 27. 
Eulamellibranchia, 232. 



3o6 



INDEX. 



Euornithes, 297. 
Eurhipidurae, 297. 
Euryalidea, 261. 
Eutheria, 298. 
Euthyneurous, 250. 
Excretion, 7, 63. 
Exopodite, 195. 
Bye, 55-59. 
Fat globules, 59. 
Fertilization, theory of, 73. 
Filibranchia, 231. 
Fission, 68. 
Flagellata, 21. 
Flagellum, 45. 
Function, 52. 
Furrow, 36. 
Galeritidse, 260. 
Gametae, 27. 
Ganglion, 47. 
Gastraae, 15, 35. 
Gastrseada, 76. 
Gastroccelom, 37. 
Gastropoda, 223. 
Gastrotricha, 117. 
Gastrula, 30, 35, 39. 
Germinal layer, 37. 
Germ plasm, 72. 
Gigantostraca, 178. 
Gill, 63. 
Glands, 60. 
Glires, 299. 
Globigerina, 20. 
Gnatliobdellidae, 109. 
Gonochorism, 30. 
Goriidse, 107. 
Gregarinida, 21. 
Growth, 7. 
Haeckel, 6, 7, 12, 15. 
Halichondrinae, 81. 
Hatschek, 12, 16, 43, 73. 



Heider, 275. 
Heliozoa, 20. 
Helminthophya, 275. 
Hemiaspidae, 178. 
Hemichorda, 275. 
Hemimyaria, 283. 
Heredity, 71. 
Hermaphroditism, 30, 67. 
Heteraxonia, 41. 
Heterogeny, 70. 
Heteromyaria, 248. 
Heteroptera, 186. 
Heteratricha, 22. 
Hexactinellidae, 77. 
Hexapoda, 182. 
Hilaire, Geoffrey St., 10. 
Hinge, 243. 
Hirudinei, 108. 
Histology, 44. 
Holocephali, 292. 
Holothuroidea, 263. 
Holotricha, 22. 
Homodynamous, 14. 
Homology, 13. 
Homophyly, 13. 
Homoptera, 186. 
Homotypical, 14. 
Hoplonemertini, 105. 
Hydrocoralliae, 81. 
Hydridae, 81. 
Hydroniedusae, 8r. 
Hydropolyp, 88. 
Hydrozoa, 81. 
Hymenoptera, 188. 
Hyperina, 173. 
Hyperoarti, 291. 
Hyperotreti, 291. , 
Hypotricha, 22. 
Ichthyopterygia, 296. 
Ichthyotomi, 202, 



INDEX. 



307 



Impennes, 297. 
Iinperforata, 19. 
Individual, 5. 
Infusoria, 22. 
Insecta, 182. 
Interambulacral, 267. 
Intracellular, 60. 
Invasion, polar, 38. 
Irritability, 6. 

Iris, 55- 
Isopoda, 172. 
Isospore, 26. 
Kidneys, 64. 
Kohler, 276. 
Korschelt, 275. 
Kowalewsky, 12, 277. 
Laemodipoda, 173. 
Lamellibranchia, 230. 
Lamprey, 291. 
Lang, 12. 
Larvacea, 278. 
Lemnisci, 129. 
Lens, 56. 
Lepadidae, 169. 
Leptocardii, 291. 
Leptostraca, 171. 
Leuconidae, 78. 
Leukart, 76. 
Liuguatulida, 193. 
Linnaeus, 8. 
Locomotion, 65. 
Lung, 63. 
Lysopteri, 293. 
Macrospore, 26. 
Macrura, 176. 
Madreporaria, 85. 
Madreporic plate, 268. 
Malacobdellini, 105. 
Malacostraca, 170. 
Mammalia, 297. 



Mandibles, 196. 
Marsipobrauchii, 291. 
Marsupialia, 298. 
Maxillae, 196. 
Maxillipeds, 196. 
Medulla, 50. 
Membrane, 24, 61. 
Meroblastic, 38. 
Mesenchyma, 42. 
Mesoderm, 37, 40, 66. 
Metabolism, 6. 
Metakinesis, 33. 
Metameres, 137. 
Metamorphosis, 67, 213, 214. 
Metapodium, 245. 
Metazoa, 7, 29. 
Metschnikoff, 76, 93. 
Microspore, 26. 
Mollusca, 221. 
Monera, 6, 19. 
Monocondylia, 295. 
Monodelphia, 298. 
Monomyaria, 246- 
Monotocardia, 224. 
Monotremata, 298. 
Monozoa, 97. 
Morphology, 12. 
Motion, 6. 
Miiller, Joh., 278. 
Muscular fibrils, 46, 47. 
Musivian eye, 57. 
Myacea, 233. 
Myoblast, 46. 
Myophrysks, 24. 
Myriopoda, 181. 
Myxopodia, 23. 
Myzostomida, 112. 
Nageli, 71. 
Narcomedusae, 83. 
Nasellaria, 20. 



308 



INDEX. 



Nauplius, 195. 
Nematocyst, 60. 
Nemathelmia, 105. 
Nematodes, 106. 
Nemertini, 104. 
Nephridium, 63. 
Nerve epithelia, 50. 
Nerve fibril, 47, 48. 
Nervous system, central, 49. 
Neuroptera, 186. 
Notochord, 289. 
Non-Calcarea, 77. 
Nucleus, 5. 
Nuda, 87. 

Nudibranchia, 229. 
Octopoda, 235. 
Odouata, 184. 
Oligochaeta, 109. 
Ontogeny, 15. 
Opheliacea, no. 
Ophiuridea, 261. 
Ophiuroidea, 260. 
Opisthobranchiata, 226. 
Orthonectidse, 77. 
Osculosa, 20. 
Osphradium, 237. 
Ossicula, 266. 
Ostracophori, 291. 
Ovum, 30. 
Paedogenesis, 68. 
Palingenetic, 15. 
Pallium, 236. 
Panorpata, 186. 
Pantopoda, 179. 
Paranucleus, 25. 
Parapodium, 245. 
Parthenogenesis, 68. 
Pauropoda, 182. 
Pedicellarise, 262. 
Pedipalpi, 191. 



Pellucida, 58. 
Perforat 20. 
Peripatus, 180. 
Periproct, 142. 
Peritricha, 22. 
Peromedusae, 86. 
Person, 29. 
Peticulidas, 186. 
Phalangidea, 191. 
Phceodaria, 21. 
Pholodacea, 233. 
Phoronidea, 113. 
Phyllopoda, 168. 
Phylogeny, 14. 
Physemaria, 76. 
Physiology, 12. 
Phytophthires, 185. 
Pigment, 59. 
Pilidium, 123. 
Pisces, 291. 
Placophora, 222. 
Plathelminthes, 95. 
Plecoptera, 184. 
Plumularia, 82. 
Pluteus, 273. 
Podopterjrgia, 293. 
Polar body, 33. 
Pole, 35. 

Polian vesicles, 268. 
Polychaeta, no. 
Polycystidea, 22. 
Polymorphism. 29. 
Polyzoa, 97. 
Porifera, 77. 
Porulosa, 20. 
Priapulidas, 113. 
Proboscidea, 300. 
Propodium, 245. 
Prosobranchia, 223. 
Prosoma, 135. 



INDEX. 



309 



Prosopygii, 112. 
Prostate gland, 101. 
Prostomium, 142. 
Protaxonia, 41. 
Proteida, 294. 
Protobranchia, 231. 
Protomerit, 24. 
Protoplasm, 5. 
Protopodite, 195. 
Prototheria, 298. 
Protozoan, 7, 19. 
Protracheata, 180. 
Protrochozoon, 120. 
Protrochula, 103. 
Pseudosauria, 295. 
Pseudoscorpionidae, 190. 
Pseud olamellibranchia, 231. 
Pterobrauchia, 114. 
Pterygota, 183. 
Pulmonata, 225. 
Quadrumana, 300. 
Race, 17. 
Radiolaria, 20. 
Radula, 247. 
Ratitae, 297. 
Ray, 8. 

Regeneration, 69. 
Reproduction, 7, 66. 
Reptilia, 295. 
Retina, 55. 
Retinulae, 57. 
Rhabdoccelidae, 95. 
Rhipidopterygia, 293. 
Rhizocephala, 170. 
Rhizopoda, 19. 
Rhynchobdellidae, 109. 
Rhynchocephalia, 296. 
Rhynchota, 185: 
Rotatoria, 116. 
Rotulae, 269. 



Ryder, 181, 182. 
Salientia, 295. 
Sarcolemma, 47. 
Sarcodina, 19 
Saururae, 297. 
Scaphopoda, 230. 
Schizonemertini, 105. 
Schizopoda, 175. 
Schwann's sheath, 48. 
Sclera, 55. 
Scolecida, 103. 
Scorpion idae, 190. 
Scyphomedusae, 85. 
Scyphozoa, 84. 
Secretion, 60. 
Sedentaria, no. 
Segmentation. 34, 38, 40. 
Selachii, 292. 
Selenka, 273. 
Sense, organs of, 51, 53. 
Septibranchia, 234. 
Seroso, 219. 
Sertulariae, 82. 
Shell, 24. 
Siphonanthae, 83. 
Siphonaterae, 187. 
Siphon ophorae, 83. 
Sipunculacea, 113. 
Sirenia, 298. 
Sirenoidea, 292. 
Solpugidae, 190. 
Somatic layer, 43. 
Spatangidea, 260. 
Species, 17. 
Spermatoblast, 32. 
Sperm atocytae, 32. 
Spermatogony, 32. 
Spermatozoa, 30, 32. 
Sphaeridia, 267. 
Spiculispongiae, 77. 



3io 



INt>£X. 



Spindles, 33. 
Splanchnic layer, 43. 
Spumellaria, 20. 
Squamata, 296. 
Stauroniedusae, 86. 
Stegocephali, 294. 
Stenoglossa, 225. 
Sterroblastula, 38. 
Sterrogastrula, 38. 
Stigmata, 25. 
Stolo prolifer, 288. 
Stomatopoda, 174. 
Streak, mesodermal, 42. 
Streptoneurous, 250. 
Strongylidae, 107. 
Stylommatophora, 226. 
Submytilacea, 232. 
Subspecies, 17. 
Suctoria, 22. 
Swarmspores, 26. 
Syconidae, 77. 
Symphyla, 181. 
Systema Naturae, 9. 
Taenia, 97. 
Taenioglossa, 225. 
Taxeopoda, 300. 
Tectibranchiata, 227. 
Teleostomi, 993. 
Tellinacea, 233. 
Telolecithal, 40. 
Tentaculata, 87. 
Terabrautia, 188. 
Tesselata, 257. 
Testicardines, 115. 
Testudinata, 296. 
Tetrabranchiata, 234. 
Tetrapneumona, 264. 
Thaliacea, 282. 



Thoracostraca, 174. 
Thysanoptera, 185. 
Thysanura, 183. 
Tornaria, 278. 
Toxodontia, 300. 
Trachea, 206. 
Tracheata, 180. 
Trachomedusae, 83. 
Trachystomata, 295. 
Trematoda, 96. 
Trichina, 107. 
Trichocystes, 25. 
Trichoplax, 77. 
Trichoptera, 186. 
Tricladidea, 95. 
Trilobita, 177. 
Trivium, 266. 
Trochophora, 118. 
Trochosphaera, 120. 
Tubularia, 81. 
Tunicata, 278. 
Urodela, 294. 
Vacuoles, 25, 59. 
Variety, 17. 
Vascular system, 270. 
Vermes, 104. 
Vertebrata, 290. 
Volvox, 27. 
Weissmanu, 71. 
Whitman, CO., 76. 
Wolf, F. C, 71. 
Xiphosura, 178. 
Zelinka, 130. 
Zeugobranchia, 223. 
Zoea, 204. 
Zonula Zinnii, 56. 
Zoology, 5. 
Zygoneura, 103. 



H 






Jr., 



iflfc 






■ 



